Grip a chin up or pull up bar with a firm overhand grip. Hang from the bar with your legs straight. Raise your legs by flexing your hips forward and bend your knees, bringing them up towards your chest. Don’t swing your body to use momentum. Use your abdominals to pull your knees to your chest. Return to the starting position, lowering your legs slowly until they are straight. Repeat the movement but this time keep your legs straight throughout the movement, as you raise them up level with your hips. Keep alternating between bent knees and straight legs for the required number of reps (or time).