Lie supine on mat with lower legs on bench. Hold plate on chest with both hands.
Flex waist to raise upper torso from mat. Keep low back on mat and raise torso up as high as possible. Return until back of shoulders contact mat. Repeat.
Exercise can be performed without added weight until more resistance is needed. Leg elevation keeps pelvis tilted back keeping low back on mat. See Arm Position During Waist Exercises and Spot Reduction Myth.