Start by hanging on a pull-up bar with palms facing out, or use the ab straps and hang with your arms inside of them. Do use a box or bench if you need help reaching the bar.
Pull your shoulder blades down your back to keep your shoulders away from your ears.
Engage your abs as you fully extend both legs, lifting them up simultaneously with control.
Lower the legs down slowly and return to the starting position. This completes one rep.
Avoid swinging during this move to ensure that you're really working your abs.
Do two sets of 10 reps.
If this move is too hard, you can do double leg lifts on the ground.