Do not swing your legs up and allow them to drop quickly.
The exercise should be slow and controlled, using your abdominals to move your legs.
Instructions
Grab a decline bench and set the angle to around 30 to 45 degrees.
Lay on the bench backwards, with your head the highest end. Hold onto the pads above your head for stability.
With your feet and knees together, point your legs out straight so your body forms a straight plank. Your legs should not be resting on the bench. This is the starting position for the exercise.
Keeping your legs straight, raise your right leg up as far as possible. Don't pause at the top, and lower your right leg back to the starting position.
Repeat this process for your left leg. That is one rep.