Breath play is a staple of BDSM play, but choking has become more and more common in recent years, even in vanilla sex. There are biological reasons for this kind of play, cutting off some of the oxygen going to the brain intensifies pleasure, but this activity is very dangerous for obvious reasons.
Usually the dominating partner restricts the airflow of the submissive partner, but this should only be done when there is a high level of trust in the relationship. The dominating partner should also have specific knowledge of any biological variables involved.
There are different techniques for restricting the breath, such as covering the mouth with a hand, face sitting and toys such as tubes or ropes.
Some people auto-asphyxiate while masturbating- same rule applies: know your shit and have supervised experience before fucking with your breathing, since things can go very, very wrong, very, very quickly.
More about breath play here:
BDSM, Bizarre, Submission, Domination
serenaa posted an article in Fucking