AKA Inverted plank
Place the upper back and lower legs on elevated surfaces such as benches, boxes or stacked plates so that the body in between these points is unsupported. Use the back and glutes to raise the hips up until the body is held in a straight line, and maintain this straight position rigidly for the prescribed period of time. Weight can be added by holding plates or other implements on the hips.
The back plank is a simple but effective exercise to strengthen the glutes and the ability of spinal extensors to maintain back extension against resistance.
The back plank should generally be held for 20-60 seconds per set, but shorter (8-15 seconds) weighted planks are also effective.