The man and woman (or, I suppose, it could happen to homos, too) are going at it pretty hard, the man withdrawals a little too far and the penis 'slips' out, as he thrusts back in he meets resistance from the perineum and the penis is 'bent' sharply, rupturing the blood-engorged vessels. Reported to be an excruciating injury to which microvascular surgical repair is a virtual certainty, though the penis will probably never be 100% again
I have heard that you jimmy gets "bent" from this happening - ouchy!