Lie supine on bench or mat. Place hands under lower buttock on each side to support pelvis. Raise one leg up vertically with knee nearly straight. If lying on floor, raise other leg slightly off of floor.
Keeping knees nearly straight, simultaneously change positions of legs so vertical leg is lowered while lower leg is raised vertically. Continue alternating leg positions.
Rectus Abdominis and Obliques only contract dynamically if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only act to stabilize pelvis and waist during hip flexion. It may be necessary to completely flex hips before waist flexion is possible. Conversely, it may be necessary to completely flex hips before waist flexion is possible, which is not practical with alternating leg movements. Also see Spot Reduction Myth and Lower Ab Myth.
To decrease intensity, flex knees along with hips. Alternatively, perform exercise on floor and allow alternating heels to make contact with floor each repetition.
To increase resistance use ankle weights or perform on incline board. Raising legs up only short height challenges greater isometric-like endurance.