Movement:
This is quite an advanced exercise and should only be performed if you are strong enough to hold your body still with most of the weight on your elbows as they press into the pads.
With a slight forward tilt and knees bent at a 90 degree angle, raise them up to your chest and as you lower them down make your legs straight. As you crunch up again make sure your legs are at a 90 degree bend as you raise them to your chest.
Always be in control of the movement.