When during Anal intercourse (the woman may need to have diarrhea) the male pulls out and smacks his dick on the females back near the tramp stamp area. This will cause a lot of the fecal matter to be flung off and leave a trail of shit up the spine.
you dude i gave that bitch the muddy bicycle tire last night and she spent an hour in the shower trying to get the shit off! hahaha she couldn't reach it!!