Sit across a flat bench with your upperbody leaning back and your legs fully extended out in front of you. Place your hands on the front of the bench to secure your body in place. With your lower back rounded, bring your upper body forward and simultaneously curl your legs in until your knees are touching your chest. Squeeze your abs in this position and then return back to the starting position.
Make sure to keep your lower back rounded as you perform your reps rather than arched. It is also very important to keep the tension on your abs throughout the entire exercise by not allowing yourself to rest at any point and by utilizing a complete range of motion.