Sit on end of bench with both legs over padded lever and hips aligned with fulcrum. Strap legs onto padded lever. Lie supine on bench. Grasp handles near head.
Raise legs by flexing hips until they are completely flexed. Lower legs until hips are extended. Repeat.
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. Also see Spot Reduction Myth and Lower Ab Myth.