One who removes an arm from a long sleeved shirt whilst grabbing the empty sleeve with his other hand and making a pumping motion with it. The removed arm remains underneath the shirt and is used to thrust through the hole created between the two sleeves.
Pumpers often travel in packs and exclusively listen to KRAT
Pumpers rarely pump it down low, they mostly try to pump it up.
Look at the action he gets when he pumps in that sweatshirt, that guy is a pro pumper, no doubt about it...