When you take a turkey baster and fill it with Boston cream, then stick it up somebody's ass, and when the ass is fully loaded with Boston cream you squeez their butt cheeks together and suck the boston cream out using your mouth.
Karps dad comes enters karps room and says "hey son u wanna boston cream Steven." Karp responds "Oh Steven of course i do." Karp jams the baster up Stevens ass And fills it with the yummy cream. He then squeezes the cheeks together and sucks out the wonderfull sensation.