When a man gets bored with regular anal and decides to slide his nuts into the pussy. This is a suave move with a few stipulations. The chick's vagina should be close to her ass. Otherwise you're tugging on your ballsack. Never let her help, women have nails and can't feel the pain. And once you're in, don't jackhammer, you have to rock in a smooth motion or start again.
Thailand, home of the Dirty Charlie Brown.