The scientists of Hatha Yoga developed an understanding of the body and its interaction with the life-energy that relies on the subtle energy centers rather than solely on physical organ systems. The physical organ systems are part of the physical body’s structural mechanism, yet without the movement of the life-force, they are dead and unable to do anything. Everything relies on this life-force, which the ancient practitioners called “prana”. In their view, there is a universal life-force which permeates all existence and carries out activity. In the physical body, it is this force that makes the difference between “life” and “death” of the body. This is a subtle force, however, and not directly perceptible by the physical senses. The practitioners of Hatha Yoga felt that by gaining mastery over the most accessible manifestation of the life-force, the breathing process, they would be able to gain control over the entire range of subtle energy actions within the body.
Sri Aurobindo provides an overview of this understanding: “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 Nadis or nerve-channels of his system. He becomes aware of its action in the six Chakras 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.”
Western scientists have done research on various claims of the Hathayogins and found that in fact, control over heartbeat, blood pressure, stress reduction and breathing could be effected by those who were proficient in the practices. Some of these are the “involuntary” functions that cannot normally be controlled by most people consciously.
This basis is a preparation for the higher aims sought by the Hathayogins, namely, the achievement of various psychic and spiritual realisations. Sri Aurobindo comments that at this point the action of Hatha Yoga begins to correspond more with the practices known as Raja Yoga, and thus, a review of Raja Yoga will be in order.
Sri Aurobindo, The Synthesis of Yoga, Part Two: The Yoga of Integral Knowledge, Chapter 27, Hathayoga, pp. 512-513