Some people enjoy playing with cutting off their air during heavy scenes. This can be as simple as squeezing someone’s neck while you kiss them deeply, 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. One simple way to start is to try squeezing your lover’s neck gently as you make love to them. If it feels good, they will let you know, most demonstratively. And you can stop instantly just by letting go. In _any_ form of breath control, it is critical that all equipment be fail-safe, and that the bottom’s breathing is only impaired by the top’s _direct_ action–not by anything (noose, gas mask, etc) that would continue to obstruct air if the top (for example) fainted suddenly.
Many people die each year practicing “autoerotic asphyxiation”–wherein someone will masturbate while restricting their own breathing, and one night they wait too long to take the bag off their head or release the pressure on their neck, and they black out and die. Some think, “Well, just play with a partner, then, if you want to black out.” However, losing consciousness, even for a moment, _can_ trigger cardiac arrest. This is why making your bottom black out is almost certainly a much riskier idea than you would think.
The same goes for anesthesia. Sometimes people think, “Hmm, it’d be hot if I could drug my play partner–like in the movies–and she’d wake up all bound!” Even if your play partner likes this idea, don’t do it. There is no safe way to force someone into unconsciousness; anesthesiologists spend their lives learning how to do it, with the best equipment, and still mishaps occur. Don’t play with ether, or chloroform, or suffocation to unconsciousness… unless you and your partner really want to take a substantial risk of death. More experienced people than you have died.