Breathing
I use breathing to regulate emotional temperature. It can calm you or raise you up if you are feeling too flat footed for any reason. I compete my best with just a little tension.
On to breathing while actually acting, it varies quite a bit depending on what you are doing. An across the course shooter that may be shooting twelve hundred yards without a rest wears a jacket that tightly constricts them, straps to the gun with a sling that locks gun and shooter together, and the shooter controls not only their breathing but in many cases their heartbeat! Yes, it is possible to gain some control over that "involuntary" muscle. Some are documented to stop their heart while breaking a shot.
I have competed at benchrest where five shots into a quarter inch center to center at 100 yards is known as a "big ugly". At a big event you just lost any chance of a very high finish in most cases. I have competed speed shooting pistols too. Set local records, won my share. Shot long range pistol and slow fire both pretty well but not in formal competitions. Raced circle track. Drag raced, where you need to release the car a split second before the green so that the green and the car moving are simultaneous. A few other things too. Playing football, waiting for the snap, do you hold your breath? Waiting on the pitch playing ball?
Some things can benefit from holding your breath at the proper moment. However, that is the catch for most of us. I break a shot in pool when everything feels right. I always take a few practice strokes, sometimes more until the shot feels right.
There is the catch with stopping breathing. There is absolutely nothing worse than running out of oxygen and I think we have all been there. I have tried many different breathing techniques during competition and I have found that a handful of slightly deeper than normal breaths before time to act, then breathing in slow not quite full depth breaths while acting works best for me. Doing things any other way I can run out of oxygen. Running out of oxygen and recovering may take a minute or two. Even getting up and back down on a shot is not likely to give time to properly oxygenate again.
We shoot in a constricted position as things are. Trying to hold our breath is a mistake for most of us I believe. If someone is trained to do this like an across the course shooter, then taking that training to pool might help a little. I think I, and most, benefit from breathing naturally before getting down on a shot or by breathing slightly deeper for a handful of breaths. We aren't trying to flood our system with oxygen either. Then gentle breaths while on the shot. These can be slightly shallow.
Works for me, as always, other's mileage may vary!
Hu