Sit on incline board. Lie supine on incline board with torso elevated. Grasp feet rollers or sides of board for support.
With knees straight, raise legs by flexing hips until hips 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. Attempting to perform Incline Straight Leg-Hip Raise at a low incline would not load waist and hip flexors after they travel beyond vertical so that sort of movement would need to be performed on steep incline or vertical position, requiring much greater strength. Also see Spot Reduction Myth and Lower Ab Myth.
Lower incline or lie on floor or bench to decrease intensity. Exercise can also be performed by bending knees and hips.
Elevate incline or straighten knees more to increase resistance. Once highest incline has been achieved, exercise can be performed vertically. Knees can also be raised to shoulders to dynamically involve Rectus Abdominis. Exercise can also be performed with added weight.