Get into proper position by locking your arms in a fixed overhead position. You can do this on a bench, with your hand next to your head and gripping the sides of the bench, or you can grip a stable object overhead. Some people use kettlebells or a barbell, but using something fixed and stable is a bit more practical, and quite a bit safer.
Contract your torso, bend your legs and drive your legs up as if you are performing a reverse crunch, but keep your body rigid from your shoulders to toes, and avoid bending the hips. Point your toes to help maintain proper body alignment.
Slowly straighten your legs while maintaining previous position. Hold for a second and get back to starting position.