Sit on incline board. Place weight between ankles. Lie supine on incline board with torso elevated. Grasp feet hooks or sides of board for support.
With knees straight, raise legs by flexing hips until thighs are just past perpendicular to torso. 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; see Weighted Incline Leg-Hip Raise. Care should be taken not to have weight falling over or thrown onto sensive bodily areas. For this reason increasing incline to increase intensity may be wiser than adding additional weight.
Exercise can be performed without added weight on greater incline. Knees can flex along with hips to decrease intensity; see Weighted Incline Leg Raise. Also see Spot Reduction Myth and Lower Ab Myth.