to show how the abs can be worked, go to a pull up bar with a spotter. have them hold your legs bent while you hang directly below the bar. do 5 or 10 pull ups. now have them pull your legs back about a foot or more and do 5 pull ups. usually i can feel my abs strain pretty hard. kiping pull ups work a lot of your core but can be dangerous on temporary pull up bars attached to door frames.