That’s one of the ways Prius keeps good gas mileage in stop-and-go traffic. It can shut of the gas engine any time it’s not needed, and start it up seemlessly and without damage, because it can pre-spin the engine using the electric motor.
My engine is always off at lights (once the car is warmed up). So on my very short, stop-and-go drive from home to work (3 miles) I get over 45 mpg. An ordinary car would be lucky to be up to 15 mpg on that trip, especially if you catch the morning commuter train and all the red lights.
This makes me wonder - does anybody think that the traffic control "experts" will ever get their act together and actually time traffic lights so that traffic doesn't have to stop every 1000 feet for a red one in some cities? This is a huge waste of energy and time, but localities love to use traffic signals as speed-control devices.
There were streets in Cleveland, Ohio, some 40 years ago that had timed traffic lights on them. Streets like Madison Avenue and Carnagie Avenue - you could drive 35 mph for several miles and always hit all the lights green. That should be the goal today, but it's not.