Most movement rules fall apart when they're questioned. In modern day Pilates, it's common to be told that you should go up to a bridge to a point where your lumbar spine appears flat in order to move "correctly". But if you go to the yoga class down the street, you're likely told to do a bridge as high as you can (lumbar extension). Shoulder Bridge in "Return to Life" requires full lumbar extension. It's possible that these are all correct, and they simply mean different things.