When you have a severe case of diarrhea, possibly from an all night drinking binge. What you do is pull your pants down and sneak up on an unsuspecting victim. Awake or asleep it doesen't matter. Aim your bare ass at them and squeeze your poop hole as tight as you can. At the same time push out with all your might and spray the unsuspecting victim with waterlike diarrhea. This is a Rectal Expulsion. Make sure you are ready to run, for when the victim realizes what has happened they will most likely try to kick your ass.
I gave my roomate a rectal expulsion this morning after i threw up.