Hyperventilating tricks the body into thinking there is no carbon dioxide building up in your lungs. You know the stuff that makes you go oh shit I need to breathe. The problem with doing this, it doesn't trigger your natural reaction to surface, people oftentimes pass out underwater IE "shallow water blackout." Special Operations training involvles a lot of under water swimming and hyperventilating is strictly prohibited by the instructors. I guess a lot of people die from shallow water blackout. So if you are trying to hold your breath for a while I reccomend trying to figure out the mental side and don't hyperventilate.