The Revers Press Up is an amazing workout for the abs. Lay on the floor with knees up and stabilize yourself by holding something heavy behind your head. Then in a smooth movement pull your legs up and push your butt up into the air. Your legs should be straight up in the air, butt off the ground. Once legs are straight up, hold for one second, then come down and return to starting position.
The key to this exercise is to control your decent. Don't just fall back to the starting position. Slowly bring your butt back to the ground and control your legs as you lower them back down.