Since we are back to the original question of pranayama, here is a new dimension to consider. Prana that is adopted by the body (and exists in five different grades of purity) is derived from the infinite cosmic life-force of the universe. It needs replenishment. Similar to the air-breathing cycle, there is a prana-breathing cycle that brings about its exchange with the life-force.
Ordinarily, prana-breathing remains a sub-conscious activity. Pranayama makes it alive, regulates it and connects it with air-breathing. Prana being energy sustains our subtle bodies, manomaya kosha etc. Though in the initial phase of reactivating the prana cycles, air-breathing couples itself with prana-breathing and drives it, eventually prana-cycle turns by itself like a fly-wheel. This makes air-breathing redundant for a while, to allow air retention (kumbhaka) either inside or outside. Kumbhaka has to happen, any deliberate one is with risks.
?When the posture has been conquered, then the motion of the prana is to be broken and controlled, and thus we come to pranayama; the controlling of the vital forces of the body. Prana is not breath, though it is usually so translated. It is the sum total of the cosmic energy. It is the energy that is in each body, and its most apparent manifestation is the motion of the lungs. This motion is caused by the prana drawing in the breath and is what we seek to control in pranayama. We begin by controlling the breath, as the easiest way of controlling the prana.? (Vivekananda, pg 214)
However, this is really advanced and should be practiced only under a teacher’s watch. Until, this stage is reached, other pranayama exercises are mainly breathing ones.
Though some people appear allergic to sutras, it is recommended for spiritual attainment that asana, pranayama, yama and niyama which are extremely complementary are secondary means of yoga. For reaching the break-through point of pratyahara, these four need to be practiced concurrently.