This is a trick for kids: you offer somebody a quarter if they can push it through your arm. You place your left hand on your shoulder, place the quarter on your left forearm and cover it with your right. Ask the kid to push, hard: then after a few seconds you say "There it goes!" You shake your left arm and the quarter comes out of your sleeve!
The trick here is an apparently innocent bobble: you accidentally let loose the quarter after you've put it on your forearm. Pick it up with your LEFT hand and place it in your right hand, then proceed. But you actually palm the quarter in your left hand and while the pushing is going on you slip it into your sleeve.
A long-sleeved shirt is necessary for the successful execution of the trick. Left-handed persons will of course reverse the "hand-ling".