" The breath relates directly to the mind and to our prana (energy/life force), but we should not therefore imagine that as we inhale, prana simply flows into us. This is not the case. Prana enters the body in the moment when there is a positive change in the mind. Of course, our state of mind does not alter with every in-breath or out-breath; change occurs over a long period of time. If we are practicing pranayama and notice a change of mind, then prana has long before entered the body. Change of mind can be observed primarily in our relationships with other people. Relationships are the real test of whether we actually understand ourselves better."
~ T.K.V. Desikachar