A frequently asked question is what are the rules of breathing during asanas? When do we inhale? When do we exhale? Do we hold our breath?
The answers to the above question can be found by practicing one key aspect of the asana practice – enjoying what you do. When you do so, breathing comes naturally. With some practice you will notice that it is easier (and enjoyable) to inhale when you stretch/lengthen the spine (as in a back bend like Cobra) and exhale when the spine collapses (like in a seated forward bend). These guidelines are truly when you are in the dynamic or moving phase of an asana. Once you reach your final state, you would breathe well – actively and completely. There are very few asanas or parts of it during which you hold your breath.
[From an April 2006 newsletter Evolution article that I wrote]