An ancient yoga textbook, the Hatha Yoga Pradipika, offers this insight: "Breath is the key to ultimate emancipation." The Upanishads, the Hindu sacred scriptures, likewise equate prana, in the form of breath, with the universal soul. When it is done properly and when a yoga practitioner is ready, pranayama, the yogic practice of regulating and channeling one's breath, can provide a bridge between the individual self and the universal soul.
It takes regular practice to develop the ability to feel something as subtle as when the universal soul and the individual self begin to separate in the course of a breath—and that recognition is what pranayama is all about. By practicing pranayama and regulating the flow of prana with measured observation and distribution of the breath, the mind becomes still. When this happens, we can allow the energy we normally use to process the world to instead bend inward.
According to Iyengar, asana practice makes the body fit for pranayama, and pranayama practice makes the mind fit for meditation. In order for us to reach the ultimate union of our individual self with the universal soul, we must first experience dhyana, or true meditation.