Beginning (example on left) – Don’t lay all the way down. Alternate driving your knees in one at a time, keep core engaged and tight.
Intermediate (middle example) – Lay on your back and alternate driving one knee in at a time to the opposite elbow, keeping core tight.
Advanced (right example) – Lay on your back, keeping shoulders and legs off of the ground the whole time. Drive upper body to legs alternating v – ups, using your core to lift your body up each time.