Hang inverted from high bar with gravity boots on each leg. Lie supine with hands on floor over head.
Raise trunk as high as possible by flexing hips and waist. Return to starting position. Repeat.
Straight knees during this exercise will inhibit hip range of motion via passive insufficiency of hamstring causing waist flexion sooner in the movement. The Rectus Abdominis and Obliques dynamically contract only when actual waist flexion occurs. With no waist flexion, Rectus Abdominis and External Oblique will only isometrically contract to stabilize pelvis and waist during hip flexion. See Spot Reduction Myth and Lower Ab Myth.
Movement can be made easier by lowering incline, performing movement on horizontal surface, or placing arms further down away from head.
Exercise can be made more challenging by placing arm further along body or even more challenging by placing hands near head or beyond head. See Arm Position During Waist Exercises. Exercise can also be performed with additional weight.