HOW: Get set-up laying flat on your back with your knees extended and feet supported on a swiss ball, that is being stabilized by a partner. Begin the exercise by bridging up to lift your hips off the ground. While maintaining this position, rapidly alternate kicking your heels down into the ball while not letting your hips drop.
FEEL: You will feel your core muscles as well as your hamstrings and glutes working with this exercise.
COMPENSATION: Do not let your hips drop or rotate side to side.