Backseat Bullride
While having sex with a girl doggy-style, use one of your hands, and grab on to the opposite boob (left hand right boob, right hand left boob). Then, whisper in her ear, "Your sister likes it this way," and try to hang on for 10 seconds.
Man, I almost did a backseat bullride on my gf last night, I hung on for 9 seconds!