Its the type of release, or even a release at all. The extreme idea of not flipping would be no release whatsover and just holding the angle all the way. Any horizontal motion of the left wrist is not ideal, only a vertical motion (hinging and unhinging). A good release combines a vertical motion of the left wrist with a supination. This is sometimes called an impact snap. There has to be some way to get the stored angular energy out of the swing and onto the ball, but not in a way that involves a horizontal motion of the left wrist... according to many.