Read Summary of Breath
- 1 What is Breath?
- 2 Why is doing Pranayama Helpful?
- 3 How to do Pranayama?
- 4 More on Breath and Pranayama
- 5 References
What is Breath?
The true breathing is not merely the inspiration and expiration from the lungs which is merely the mechanism of it, but a drawing in of the universal energy of Prana into every cell of the body. 
...the physical being is made up of two elements, the physical and the vital, the body which is the apparent instrument and the basis, and the life energy, prāṇa, which is the power and the real instrument. Both of these instruments are now our masters. We are subject to the body, we are subject to the life energy; it is only in a very limited degree that we can, though souls, though mental beings, at all pose as their masters. 
Control of the Breath
... prāṇāyāma, the regulated direction and arrestation by exercises of breathing of the vital currents of energy in the body. 
According to Sri Aurobindo,.. true movement behind respiration is the same as the one governing electrical and magnetic fields; it is what the ancient yogis used to call Vayu, the Life-Energy. The breathing exercises (prāṇāyāma) are simply one system (among others) for acquiring mastery over Vayu which eventually enables you to be free from gravitation and gives certain powers know to the ancients: the power to be extremely light or extremely heavy, very big or very tiny (garimā, laghimā, mahimā, aṇimā). 
Why is doing Pranayama Helpful?
The exercises of Pranayama are the familiar mechanical means of freeing and getting control of the pranic energy. They heighten too and set free the psychic, mental and spiritual energies which ordinarily depend for their opportunity of action on the pranic force. But the same thing can be done by mental will and practice or by an increasing opening of ourselves to a higher spiritual power of the Shakti. 
..the first objects of the Pranayama are to purify the nervous system, to circulate the life-energy through all the nerves without obstruction, disorder or irregularity, and to acquire a complete control of its functionings, so that the mind and will of the soul inhabiting the body may be no longer subject to the body or life or their combined limitations. The power of these exercises of breathing to bring about a purified and unobstructed state of the nervous system is a known and well-established fact of our physiology. It helps also to clear the physical system, but is not entirely effective at first on all its canals and openings;therefore the Hathayogin uses supplementary physical methods for clearing them out regularly of all their accumulations. The combination of these with Asana,—particular Asanas have even an effect in destroying particular diseases,—and with Pranayama maintains perfectly the health of the body. But the principal gain is that by this purification the vital energy can be directed anywhere, to any part of the body and in any way or with any rhythm of its movement. But the principal gain is that by this purification the vital energy can be directed anywhere, to any part of the body and in any way or with any rhythm of its movement. 
The mere function of breathing into and out of the lungs is only the most sensible, outward and seizable movement of the Prana, the Breath of Life in our physical system. The Prana has according to Yogic science a fivefold movement pervading all the nervous system and the whole material body and determining all its functionings. The Hathayogin seizes on the outward movement of respiration as a sort of key which opens to him the control of all these five powers of the Prana. He becomes sensibly aware of their inner operations, mentally conscious of his whole physical life and action. He is able to direct the Prana through all the nāḍīs or nerve-channels of his system. He becomes aware of its action in the six cakras or ganglionic centres of the nervous system, and is able to open it up in each beyond its present limited, habitual and mechanical workings. He gets, in short, a perfect control of the life in the body in its most subtle nervous as well as in its grossest physical aspects, even over that in it which is at present involuntary and out of the reach of our observing consciousness and will. Thus a complete mastery of the body and the life and a free and effective use of them established upon a purification of their workings is founded as a basis for the higher aims of Hathayoga.
...some people practice pranayama with the idea of gaining "powers." That idea of gaining powers fouls it up more than anything...do it simply as a help to your progress... 
To Perfect the Body
Pranayama.. serves a double purpose. First, it completes the perfection of the body. The vitality is liberated from many of the ordinary necessities of physical Nature; robust health, prolonged youth, often an extraordinary longevity are attained. On the other hand, Pranayama awakens the coiled-up serpent of the Pranic dynamism in the vital sheath and opens to the Yogin fields of consciousness, ranges of experience, abnormal faculties denied to the ordinary human life while it puissantly intensifies such normal powers and faculties as he already possesses. 
The body,.. liberated from itself, purified from many of its disorders and irregularities, becomes, partly by Asana, completely by combined Asana and Pranayama, a perfected instrument. It is freed from its ready liability to fatigue; it acquires an immense power of health; its tendencies of decay, age and death are arrested. The Hathayogin even at an age advanced beyond the ordinary span maintains the unimpaired vigour, health and youth of the life in the body; even the appearance of physical youth is sustained for a longer time. He has a much greater power of longevity, and from his point of view, the body being the instrument, it is a matter of no small importance to preserve it long and to keep it for all that time free from impairing deficiencies. 
To Calm Oneself
Suppose for example, your heart begins to throb madly; then you must make it calm, you tell it that this is not the way to behave, and at the same time (solely to help it) you take in long, very regular rhythmic breaths, that is, the lung becomes the mentor of the heart and teaches it how to work properly.
To Awaken the Spiritual Energies
...arrangement of the psychic body is reproduced in the physical with the spinal column as a rod and the ganglionic centres as the chakras which rise up from the bottom of the column, where the lowest is attached, to the brain and find their summit in the brahmarandhra at the top of the skull. These chakras or lotuses, however, are in physical man closed or only partly open, with the consequence that only such powers and only so much of them are active in him as are sufficient for his ordinary physical life, and so much mind and soul only is at play as will accord with its need. This is the real reason, looked at from the mechanical point of view, why the embodied soul seems so dependent on the bodily and nervous life,—though the dependence is neither so complete nor so real as it seems. The whole energy of the soul is not at play in the physical body and life, the secret powers of mind are not awake in it, the bodily and nervous energies predominate. But all the while the supreme energy is there, asleep; it is said to be coiled up and slumbering like a snake,—therefore it is called the kuṇḍalinī śakti,—in the lowest of the chakras, in the mūlādhāra. When by Pranayama the division between the upper and lower prana currents in the body is dissolved, this Kundalini is struck and awakened, it uncoils itself and begins to rise upward like a fiery serpent breaking open each lotus as it ascends until the Shakti meets the Purusha in the brahmarandhra in a deep samadhi of union.
Put less symbolically, in more philosophical though perhaps less profound language, this means that the real energy of our being is lying asleep and inconscient in the depths of our vital system, and is awakened by the practice of Pranayama. In its expansion it opens up all the centres of our psychological being in which reside the powers and the consciousness of what would now be called perhaps our subliminal self; therefore as each centre of power and consciousness is opened up, we get access to successive psychological planes and are able to put ourselves in communication with the worlds or cosmic states of being which correspond to them; all the psychic powers abnormal to physical man, but natural to the soul develop in us. Finally, at the summit of the ascension, this arising and expanding energy meets with the superconscient self which sits concealed behind and above our physical and mental existence; this meeting leads to a profound samadhi of union in which our waking consciousness loses itself in the superconscient. Thus by the thorough and unremitting practice of Pranayama the Hathayogin attains in his own way the psychic and spiritual results which are pursued through more directly psychical and spiritual methods in other Yogas. The one mental aid which he conjoins with it, is the use of the mantra, sacred syllable, name or mystic formula which is of so much importance in the Indian systems of Yoga and common to them all.
The Rajayogic Pranayama purifies and clears the nervous system; it enables us to circulate the vital energy equally through the body and direct it also where we will according to need, and thus maintain a perfect health and soundness of the body and the vital being; it gives us control of all the five habitual operations of the vital energy in the system and at the same time breaks down the habitual divisions by which only the ordinary mechanical processes of the vitality are possible to the normal life. It opens entirely the six centres of the psycho-physical system and brings into the waking consciousness the power of the awakened Shakti and the light of the unveiled Purusha on each of the ascending planes. Coupled with the use of the mantra it brings the divine energy into the body and prepares for and facilitates that concentration in Samadhi which is the crown of the Rajayogic method. 
How to do Pranayama?
The higher use of Hathayoga depends more intimately on Pranayama. Asana deals more directly with the more material part of the physical totality, though here too it needs the aid of the other; Pranayama, starting from the physical immobility and self-holding which is secured by Asana, deals more directly with the subtler vital parts, the nervous system. This is done by various regulations of the breathing, starting from equality of respiration and inspiration and extending to the most diverse rhythmic regulations of both with an interval of inholding of the breath. In the end the keeping in of the breath, which has first to be done with some effort, and even its cessation become as easy and seem as natural as the constant taking in and throwing out which is its normal action. 
The time varies. ..inhale through the left nostril for let's say 4 seconds, then ... hold your breath for 16 seconds, raising the diaphragm and closing all the openings; after 16 seconds ... exhale for 8 seconds through the other nostril... ...that's the proportion: inhale 4, hold 16, exhale 8… It has to be double the exhalation. If you do 8, then it's 8-32-16. [This statement is made by a disciple on the traditional way of doing prayanama] 
You can achieve excellent control of the heart. But I never practiced it violently, never strained myself. I think holding for 16 is too long. I used to do it simply like this: breathe in very slowly to the count of 4, then hold for 4 ... lifting the diaphragm and lowering the head.., closing everything and exerting pressure (this is an almost instantaneous cure for hiccups—it's handy!). Then while I held the air, I would make it circulate with the force (because it contained force, you see) and with the peace as well; and I would concentrate it wherever there was a physical disorder (a pain or something wrong somewhere). It's very effective. The way I did it was: inhale, hold, exhale and empty—you are completely empty. It's very useful; very handy for underwater swimmers, for instance!
I had trouble breathing in slowly enough—that's a bit hard. I began with 4 and eventually managed to do 12. I did 12-12-12-12...To breathe in very slowly and hold all that air isn't easy. 
But instead of doing equal amounts of time, it might be better to do less for inhaling and more for holding the breath. The holding part is extremely interesting! When the air is inside, let's say you have a headache or a sore throat or a pain in your arm, anything—then you take the air….and direct it to the unwell part... very, very helpful and pleasant and interesting. You see the force go to the spot, settle in and stay there, all sorts of things.
Calling in Pranic Energy
The play of the pranic shakti in the body or form is the condition of all action, even of the most apparently inanimate physical action. It is the universal Prana, as the ancients knew, which in various forms sustains or drives material energy in all physical things from the electron and atom and gas up through the metal, plant, animal, physical man. To get this pranic shakti to act more freely and forcibly in the body is knowingly or unknowingly the attempt of all who strive for a greater perfection of or in the body. The ordinary man tries to command it mechanically by physical exercises and other corporeal means, the Hathayogin more greatly and flexibly, but still mechanically by Asana and Pranayama; but for our purpose it can be commanded by more subtle, essential and pliable means; first, by a will in the mind widely opening itself to and potently calling in the universal pranic shakti on which we draw and fixing its stronger presence and more powerful working in the body; secondly, by the will in the mind opening itself rather to the spirit and its power and calling in a higher pranic energy from above, a supramental pranic force; thirdly, the last step, by the highest supramental will of the spirit entering and taking up directly the task of the perfection of the body. In fact, it is always really a will within which drives and makes effective the pranic instrument even when it uses what seem to be purely physical means; but at first it is dependent on the inferior action. When we go higher, the relation is gradually reversed; it is then able to act in its own power or handle the rest only as a subordinate instrumentation. 
Side Effects of Improper Practice of Pranayama
You can write to him that it is not safe to do Pranayam except under the directions of a guru who is siddha in either Rajayoga or Hathayoga. Gasping is obviously a sign of something wrong—for the breathing in Pranayam must be perfectly unimpeded and regular. It is better either to stop the Pranayam or to find out somebody who is practised in the method and take instructions from him what to do. 
Pranayam is a very powerful thing, but if done haphazardly it may lead to the raising of obstructions and even in extreme cases illness in the body. 
Pranayam is safe only if one knows how to do it and is on guard against its possible dangers: (1) danger to health by mistakes in the method, (2) rising of the vital forces, especially lust, egoism and wrongly directed strength and force, (3) the awakening of concealed sanskaras of the physical nature or latent karma from past lives. 
More on Breath and Pranayama
Pranayama and other physical practices like asana do not necessarily root out sexual desire—sometimes by increasing enormously the vital force in the body they can even exaggerate in a rather startling way the force too of the sexual tendency, which, being at the base of the physical life, is always difficult to conquer. The one thing to do is to separate oneself from these movements, to find one's inner self and live in it; these movements will not then any longer appear as belonging to oneself but as surface impositions of the outer Prakriti upon the inner self or Purusha. They can then be more easily discarded or brought to nothing. 
My breath runs in a subtle rhythmic stream;It fills my members with a might divine: 
Read Summary of Breath
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