"and you suck..."
The first male (let's call him Mike) is to bring his girlfriend or wife to the given location at the given time. They proceed to have sex doggy style.
Mike's friend (Jimmy) silently enters the location and strips down. After a predetermined signal is given Mike and Jimmy switch positions so that Jimmy is now fucking Mike's girlfriend. This action is so fast that Mike's girlfriend does not realize who is now fucking her. Usually followed by a Manhattan Divorce.
2. As a prank: Those idiots...they gave me The Switch!