Sorry, but doing the drill this way is neuro-physiologically unsound, and will create bad habits. There are other ways of strengthening the wrist (ie, lift weights) which do not contribute to bad throwing habits. The elbow should always end up fully extended, and the fingers pointing towards the ground. Repeatedly teaching the brain that the ball is released while the elbow is at 90 degrees is counter-productive, and will contribute to poor overall throwing technique.