Lie supine on bench. Place hands under lower buttock on each side to support pelvis.
Raise legs by flexing hips and knees until thighs are completely flexed. Return until hips and knees 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. It may be necessary to completely flex hips before waist flexion is possible, as in leg-hip raise. Also see Spot Reduction Myth and Lower Ab Myth.
To decrease intensity, perform exercise on floor and allow heels to make contact with floor each repetition.
Perform on incline board or perform with knees straight to increase resistance. Knees can also be raised to shoulders to dynamically involve Rectus Abdominis. Exercise can also be performed with added weight.