Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 2, Verse 50
Pada 2, Sutra 50
Baahyaabhyantara-stambha-vrttir desha-kaala-sankhyaabhih-paridrsto deergha-sooksmah
Pranayama (Breath Control) is external, internal or suspended in its movements. It is perceived and regulated by place, time and number; it can be protracted or subtle.
The science and art of Pranayama consists of three phases: inhalation/inspiration (Puraka), exhalation/expiration (Rechaka) and retention/suspension (Kumbhaka). Within those three phases the breath can be modified by directing it to a specific location within the body (e.g. Kshetram or trigger point for entry or exit of Prana to access a Chakra), by the length of the inhalation and exhalation (time), the number of repetitions and the intensity by which the breath is drawn in and expelled (protracted or subtle).
Variations on those modifications will produce different effects physically, mentally and psychically. It is therefore essential to be instructed by a qualified Yoga instructor or bona fide Yogi in order to obtain the desired effects.
Breath is life and life is breath. Breath is Spirit and Spirit infuses life into matter through the breath. The process of breathing is infusing Spirit into matter. Everything is Consciousness in expression through Nama (name) and Rupa (form) (both gross and subtle) by the infusion of the Life-Force or Prana.
The original Life-Force is the Pranava which is expressed as the (Original) Word being the Divine Vibration OM (or Original Mind) / AUM (the All-encompassing trinity of past-present-future or Manifestation (Akaar), Preservation (Ukaar) and Dissolution (Makaar) - from the Infinite to infuse and suffuse the finite and back to the Infinite).