Usually dispensed in the school dinner queue. In order to make a Cup-a-Poop one hand was cupped over the arse crack, so that an SBD could be stealthily emanated.
The warmed and stinking cupped hand would be clenched into a fist to contain the smells. Then opened up over the mouth and nose of any unsuspecting smaller kid along with the shout of Cup-a-Poop! Cup-a-Poop! Cup-a-Poooooooooop! You would rub your hand quite hard in their face, to make sure every last drop went up their nose.
Occasionally you would ask your victim first; 'What is your favourite flavour Cup-a-Poop?' Whatever they answered would be challenged with 'Well I think it's beef' before the Cup-a-Poop was served.