I don't know about you all, but I love breath play. Without going anywhere near the danger zone, I can put a level of control and fear into the game that is hard to match. And it requires no equipment but your hand. Or hands.
I know it's controversial -- the edge is hard to define -- and so I haven't talked much about it here. I've edited out a couple of c's better exploits because she and K push breath play pretty far.
But I've found a way to use hypnosis in breath play, and I think it's above reproach for safety and sanity.
It's simple, too. You can set it up in advance with your sub's subconscious. Or, if you've set up a simple way to give new commands in mid-scene, you can slip it in then.
"In a moment, you're going to start holding your breath, while I start counting. At 10 you'll have an orgasm. At 20 you can breath again. OK ... start."
I think you'll find the results are explosive.