Let's take a deep breath, exhale, stay there ... keep your breath until the moment when your body will literally force you to inhale. This force that brings air or vital energy into your body is called Prana. Prana is the life force within us.
If there is not breath - there is no yoga. The mind and the body are joined together through the breath. We can breathe voluntary and involuntary. In the voluntary breathing, we can control the speed - slow down or speed up. The involuntary breathing is something that the body knows how to maintain. The dual nature of breathing can also show us that there are things that we can control in this life and things that we can't control but only surrender. The duality shows you your breath limits but also shows you that something that previously you though you can't control - you can actually do now. And this aspect of yoga is far more interesting then the conversation of "can I get my leg behind my head".