Forced sexual congress with another instance of one's self, either brought about through sensory replacement, reprogramming of the brain, or disconnection of the central nervous system from the brain, allowing remote control of a body, with its brain conscious and aware of its actions.
The most famous bafti occured when the time traveling criminal forces of either Wilem Alan Gallagher, or Padraig Steven Gallagher, experienced an endless attack from continuously duplicating instances of Thomas Aldous Rawling, and they reprogrammed the duplication mechanism so that the original time traveling criminal was infinitely duplicating and collating instances of himself, and recoding his brain to have irrepressible sexual lust for himself, and an unwavering enthusiasm for engaging in the bafti, until those new instances of him would eventually waste away (probably from dehydration). It spread throughout the universes.