I'm not claiming I "invented" this, but haven't seen it mentioned before. If it has been written about here, forgive me. When I kiss a sub, I insist she exhale right before we kiss and hold her breath out until I break the kiss. Naturally I can breath at a normal rate. I can make the kiss last as long or as short as I want, and the time frame differs each time. Punishment is earned for breaking the kiss. What's great about this very simple technique is that it can be done in public, requires no equipment, and has a built-in safety feature. The sub, of course, can always inhale...for a price.