Classical yoga under Patanjali has 8 different "limbs" of yoga. The first "limb" is the Yamas or social contracts. The second "limb" are the Niyamas or contracts with oneself. The third "limb" is Asana or yoga poses -- there are hundreds! The fourth "limb" is Pranayama or breath!
In my opinion, Pranayama is breath, is life-force. As human beings we have a brain. Our brains have a nervous system which can be conscious and subconscious. The subconscious nervous system instructs certain actions. One such action is to tell the muscle of our diaphragm to contract pulling our diaphragm down creating a vacuum in your chest cavity which sucks air into our lungs! It literally sucks air into our lungs!
Or to look at it another way, the universe is forcing air into you to keep you alive. It literally forces air into you!