Some people enjoy playing with cutting off their air during heavy scenes. This can be as simple as tying a tie around your neck and then to the head of your bed, and putting pressure on it as you are teased or brought to orgasm; or as complex as a full-head latex hood and gas mask over a straitjacket. As your air is cut off, you can feel sensation more intensely; it is also a deeply intimate thing to allow someone else to be in control of the very air you breathe. One simple explanation is that the body’s natural reaction as orgasm approaches is shallow, rapid breathing–just like in breath control.
Needless to say, there are many things that can go very wrong; if you pass out and someone isn’t there to cut you loose and make sure you’re breathing, you can die. Not for novices. If you want to play with this by yourself, set things up so if you pass out unexpectedly the pressure will be released. Your life is depending on your rig being fail-safe, and there are many experienced people who have died doing this solo.