This move is best performed by anchoring your hands on a stable object. A couch or heavy table will work fine. Lying down face up, grab your anchor and straighten your legs. Flexing your ab muscles, bring your legs straight up and as close to your hands as possible, lifting your tailbone/hips off the ground momentarily. Then lower your legs back down, and, without touching the ground, bring your legs back up again. Always keep your legs straight.
Helpful tips and Tricks
If you cannot anchor yourself, or if it is too difficult as described, you can put your hands under your hips and perform the move more easily.