AKA Bruce Lees
The dragon flag is a difficult ab exercise popularized by and named for Bruce Lee.
Lie on your back on a bench and grip the outer edges of the bench by your head. Bring your body up in a straight line supported only by your upper back. Staying as straight as possible, lower yourself until your glutes are almost on the bench and legs close, then return to the top position, again maintaining as straight of a body as possible.
Scale the motion to progress by bending at the hips and/or bending the knees, or by only lowering partway before returning to the top.
The dragon flag can be thought of as a more advanced plank variation—the idea is to be capable of stabilizing the spine and pelvis against unwanted lower back extension against considerable resistance.
3-4 sets of max reps is a decent approach—scale the motion on at least some of your reps to try to get at least 5 per set.