When you and you're partner are amidst "doggie style" sex; right before you cum you pull out and push your partners head and shoulders off the bed onto the floor. Your partners back now is sloped from the bed to the floor. You splooge on your partners back and decide to ride it down like a waterslide.
Mark - Damn Lisa why do you have a bruise on your forehead
Lisa - Me and my boyfriend were having sex and he waterslided me! And my back hurts too.