Butt Lifts

In a forearm plank, lift your butt into the air, coming all the way onto your tip-toes. Do not force down the shoulders to get your butt into the air. You are lifting your hips straight up to the ceiling, pulling in your abs. Think of someone lifting your hips up to make an upside down V.

Takes a few tries to master it. The key is to get all the way on your tippy toes and to NOT push down your shoulders.