When two people preferably men are performing anal sex, one man ejaculates into the butt of the other. The two men then go to bed, and in the morning are awakened by a fart from the man who took it up the butt. While he farts he releases the dried up seamen from the other man and it comes out as a chalky puff of air.
Sam awoke to the sound of his lover farting in bed. "Oh gross Shane you got butt chalk all over me!!!"