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. Exhale as you do so. 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. Inhale as you do so. Repeat.