Sit on edge of bench with legs extended to floor. Place weight between ankles. Grasp edge of bench. Lean torso back and balance bodyweight on edge of bench.
Raise legs by flexing hips and knees while pulling torso slightly forward to maintain balance. Return by extending hips and knees and lean torso back to counter balance. Repeat.
Heels may make contact with floor to maintain balance at bottom of movement. 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. Exercise can be performed without added weight until more resistance is needed (See Seated Leg Raise without weight). Also see Spot Reduction Myth and Lower Ab Myth.