HOW: Start by lying on your back and bending both knees up so they are straight up from your hips. With one hand, place it on the top of the opposite knee. Slightly push into that hand with your knee with no motion as you let the other leg lower down tapping your heel on the ground and bringing it back up.
FEEL: You should feel your core and oblique muscles working.
COMPENSATION: Keep the one knee up and bent pusing into that hand, don’t let it come down like the other knee.