Lie supine on incline board with torso elevated. Grasp feet hooks or sides of board by head for support.
Raise legs by flexing hips and knees until hips are fully flexed. Continue to raise knees toward shoulders by flexing waist, raising hips from board. Return until waist, hips and knees are extended. Repeat.
When raising hips, keep knees fully flexed, so as not to throw weight of lower legs over head. Rectus Abdominis and Obliques dynamically contract only if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only isometrically contract to stabilize pelvis and waist during hip flexion. It may be necessary to completely flex hips before waist flexion is possible. See Spot Reduction Myth and Lower Ab Myth. Also see Incline Leg Raise.
Exercise can be made easier by decreasing decline or lying on horizontal surface.
Exercise can be performed with additional weight. Elevate incline or perform in vertical orientation to increase resistance.