The fundamental principle of all energy

Everything is about increasing energy prod + preserving/maintaining/holding it + releasing the excess/waste product.

All kriya, yoga practice, exercise, food, breathing, supplements, vitamins.

All to facilitate greater prana/energy/life-force production. 

Purak, Kumbhaka -> Rechak, Shunyaka = [in, in-hold] -> [out, out-hold] = yin, yang = gathering and holding potency -> releasing and empty-stability.

This is the fundamental principle of all energy.