donkey punch
n.— «The Donkey Punch is sometimes employed during gay sex acts. When going at it for a while the bottom of the “bottom” can get too relaxed, so the “top” can lose enjoyment. To orgasm, the man on top punches his partner really hard in the kidneys…so that he involuntarily clenches with the shock and pain…and the guy on top gets his rocks off.» —Popbitch (U.K.) Apr. 6, 2005. (source: Double-Tongued Dictionary)