When positioned doggy style behind your partner, and you generate friction with your cock between her ass cheeks to make you cum all over her back.
My girlfriend was reading in bed last night, and didn't want to fool around. I was really in the mood, so she let me give her the Hammer Backside Slide. I totally blew my load all over her back. She was in the mood then, but she was too late. When she went to take a shower, I fell asleep.