Hook feet under padding and lie supine on incline bench with hips bent. Hold plate on upper chest with both hands or use no weight.
Flex and twist waist to raise upper torso from bench to one side. Return until back of shoulders contact padded incline board. Repeat to opposite side alternating twists.
Exercise can be performed without added weight until more resistance is needed. See Arm Position During Waist Exercises. Elevate incline to increase resistance. If no knee support is built in to incline board, ball can be placed under legs as illustrated.
Hip and knee flexors may be involved as stabilizers if incline is steep and no calf support is used. See Spot Reduction Myth.