Breathplay, the most trusting of disciplines for the sub. He has to have complete confidence in his Mistress, confidence that she will take him to the edge but return him unharmed. Now combine this with giving sub something else to distract him, like needles through his bell end. Now we have the ultimate thrill, slave having to concentrate on breathing slowly as to not run out of air, combined with the excruciating pain of the needles.