HOW: Get set-up in a hook lying position laying on your back with your knees bent. Flatten your low back into the ground to engage your core, then lift one leg up at a time to to bring your knees up above your stomach. Lower down and repeat on the opposite side.
FEEL: You should feel your abdomen muscles working as well as the muscles in front of your hips.
COMPENSATION: If you're having trouble keeping your low back flat, push your opposite foot into the ground when you go to lift the other leg. Do not let your back arch.