Correct me if I'm wrong: When you reduce the power rapidly and subsequently, the engine cools rapidly, the engine may be damaged. So it obviously makes sense to reduce the power slowly. However, if you consider the drop in temperature that's incurred when switching off the engine, it is just as great, if not greater and occurs over a shorter period of time, so presumably this would cause the most damage and make shock cooling unavoidable if it really was a serious problem.