Prana and Pranayama
Pranayama is an exact science. It is the fourth Anga or limb of Ashtanga Yoga. "Tasmin Sati Svasaprasvasayor- gativicchedah Pranayamah"-Regulation of breath or the control of Prana is the stoppage of inhalation and exhalation, which follows after securing that steadiness of posture or seat, Asana. Thus is Pranayama defined in Patanjali Yoga Sutras, Ch. II-49.
'Svasa' means inspiratory breath and 'Prasvasa' is expiratory breath. Breath is the external manifestation of Prana, the vital force. Breath, like electricity, is gross Prana. Breath is Sthula, gross. Prana is Sukshma, subtle. By exercising control over this breathing you can control the subtle Prana inside. Control of Prana means control of Mind. Mind cannot operate without the help of Prana. The vibrations of Prana only produce thoughts in the mind. It is Prana that moves the mind. It is Prana that sets the mind in motion. It is the Sukshma Prana or Psychic Prana that is intimately connected with the mind. This breath represents the important fly-wheel of an engine. Just as the other wheels stop when the driver stops the fly-wheel, so also other organs cease working when the Yogi stops the breath. If you can control the fly-wheel, you can easily control the other wheels. Likewise, if you can control the external breath, you can easily control the inner vital force, Prana. The process by which the Prana is controlled by regulation of external breath is termed Pranayama.
Just as a goldsmith removes the impurities of gold by heating it in the hot furnace, by strongly blowing the blow-pipe, so also the Yogic student removes the impurities of the body and the Indriyas by blowing his lungs, i.e., by practising Pranayama.
The chief aim of Pranayama is to unite the Prana with the Apana and take the united Prana-apana slowly towards the head. The effect or fruit of Pranayama is Udghata or awakening of the sleeping Kundalini.