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I agree with both of you.
A pitcher could of course get greater ball velocity by crashing to a stop just before he releases the ball. It hyper-accelerates the centrifiguality for him to stop suddenly as the sling is moving forward.
The problem is the loss of control and the injuries. If you were trying to stop a car, would you want your travel path to suddenly change during deceleration? A pitcher with a locked front knee has his CG abruptly veer upward as he hits the front halberd.