Sit on chair or bench with lower leg secured under low over hang or heavy bar. Place hands on waist, chest, neck, or head depending upon desired difficulty.
Lower body back until hips are almost extended. Raise body by flexing hips until torso is upright. Repeat.
Feet must be propped securely to prevent injury. Exercise can also be performed on Roman Chair Sit-up apparatus. Rectus Abdominis and Obliques only contract dynamically if actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only act to stabilize pelvis and waist during hip flexion. See Spot Reduction Myth and Lower Ab Myth. Also see Dangerous Exercise Essay.
Movement can be made easier by placing arms further down away from head.
Exercise can be made more challenging by placing hands further up toward head. See Arm Position During Waist Exercises. Exercise can also be performed with additional weight.