Hook feet under foot bar and lie supine on incline bench with hips bent. Cross arms on chest.
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.
If no knee support is built in to incline board, ball can be placed under legs. Hip and knee flexors may be involved as stabilizers if incline is steep and no calf support is used. See Spot Reduction Myth.
To make this movement easier, perform twisting crunches on lower height incline or on flat surface.
Exercise can be made more challenging by placing hands further up toward head. See Arm Position During Waist Exercises. Exercise can also be performed on higher incline board or with additional weight.