Done safely breath control can be so freakin' hot. Done incorrectly it can be fatal.

A really interesting way of playing that I just learnt this week is to lock your mouths together, one person's lungs full of air and the other person's empty. You plus each other's noses and you pass the singular breath back and forth until all you're passing is carbon dioxide. As you get going the panicky feeling of breath control sets in, and the breath gets past faster and faster, pulses raise, etc.

It gets to be a little competitive, but is exceedingly safe.