Sit on floor or mat. Lie supine with hands to sides.
Simultaneously raise knees and torso until hips and knees are flexed. Return to starting position with waist, hips and knees extended. Repeat.
Begin each repetition with upper back on floor to allow abdominal muscles to work dynamically. The Rectus Abdominis and Obliques dynamically contract only when 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. See Spot Reduction Myth and Lower Ab Myth.