While breathing is not optional, we do have choices about how we choose to breathe. Admittedly, most of the time, choosing to breathe in and then out with as little thought as possible is the most practical choice. Our wonderful bodies will do this automatically.
There are times, however, when different kinds of breathing can assist different kinds of motion. I promise to limit my discussion to two kinds, known to biomechanics as pump handle and bucket handle breathing. Feel free to make up a better name if you so desire.
Pump handle breathing describes the movement of the upper ribs and sternum. When we inhale high in our chests, our sternums lift like a pump handle, increasing the space in our chests from front to back. This kind of breathing can be useful when we want to curve our upper backs backwards (technical term: thoracic extension) while swimming, for example, without losing control of our abdominals.
Bucket handle breathing applies more to the lower ribs. The ribs are curved and at rest have the shape of a bucket handle resting on the side of a bucket, thus the clever name. When we inhale, we can lift the handle up toward the lip of the bucket, which increases the space in our thoraxes from side to side. A variation of this breathing pattern is extremely useful in finding our connection to our lower abdominals. If we breathe our side ribs out wide and then leave them wide while we exhale, we can feel our lower abdominals engage, giving us a handy focusing spot for flattening out our abdominals during exercise.
We like flat abs.