[Daryl]

Is it true that at some point the turning of the torso in the downswing/release has to stop accelerating to get the best result at impact?
If so, why?

False. If the Leading component slows, then the lagging component slows.

6-C-0 GENERAL “Loading” means establishment of the Lag, Drag and Thrust – at the selected Assembly Point (10-21) by the selected Loading Action (10-22) – calculated to produce the necessary Downstroke Thrust and Impact Force for the situation at hand. “Lag” defines the condition of “trailing,” or “following,” and can, and usually should, exist to some degree at every point in the Stroke from feet to Clubhead. Every Lagging Component places a Drag on its preceding Component, which is proportional to the Rate of Acceleration of the leading component.