The **Start-Stop** system is a technology designed to improve fuel efficiency and reduce pollution. This system automatically turns off the engine when the vehicle is stopped, such as at a traffic light or in a traffic jam. This occurs when the engine is **idling**, that is, when the vehicle is stationary and the engine is running but not accelerating. By turning off the engine in these situations, unnecessary fuel consumption and the emission of polluting gases are avoided. Therefore, the correct answer is that the Start-Stop system turns off the engine when it is idling.