Straddle machine facing handles. Grasp handles on each side and place forearms on pads. Place shins on padded sled with knees forward and feet hanging off of back end. Sit back toward heels without bending over.
Slide forward and up by pulling knees up high. Deliberately attempt to flex waist in a "C" shape. Return sled down and back arching spine in opposite direction while keeping hips bent. Repeat.
If low back does flex near top of motion, abdominal muscles may only be isometrically involved in exercise. Pectineus, Adductor Longus, and Brevis do not assist in hip flexion since hips are already initially bent. Sled pad can be turned to either side to emphasize Obliques. See Spot Reduction Myth.