Breathplay is a form of Edge play where the Domme restricts the amount of air available to the sub. Smothering in various ways, suffocation with a plastic bag, or strangling with hands or ligature are all forms of breath play.
For breath players, it is at once the most exciting, pleasurable and dangerous of all BDSM practices. As his oxygen is reduced, and reduced further, the sub may feel great fear, but he will also become deeply euphoric. The Domme experiences the ultimate sense power of having absolute control over the sub, over his breath and his life. She literally holds his life in her hands.
It is potentially dangerous. The vast majority of breath players don't experience problems. But a few do. Subs have lost their lives. Dommes have gone to jail. The danger is that if the sub loses consciousness, he may have suffered cardiac arrest. It is highly unlikely that he can be resuscitated. On the other hand he may have lost consciousness merely through oxygen depletion and he will quickly recover when he is allowed to breathe fully again. The problem is, it is impossible to know beforehand whether the sub will pass out, and the precise reason. Further, if strangling is used, there is the risk of damaging delicate structures in the neck and throat. Urgent medical attention may be required.
If sub and Domme engage in breath play, they both should be fully aware of the potential dangers. The pleasures can be extreme, but so can the consequences.