How efficiently does the Prius turn on the ICE while driving?

I assume if your foot is off the accelerator and you coast up above 46 mph in the gen III, then mg2 is acting as a generator. It might during this ice spinning simply generate less electricity to spin up the ice smoothly. I don't believe it actually starts the engine, it only turns it without adding fuel. If your foot is on the brake, I do not know what speed it will spin the ice, but assume it would do so at some higher speed to slow the car or protect mg1. These are special cases, and I don't believe I have experienced the second one, and have not paid close attention to the first case.