Shuts the engine off when the vehicle is at a standstill, in traffic or at a junction for example, restarting it when the driver engages the accelerator.
Automatically managed in hybrid cars, which typically switch to electric-only power at low speeds, diesel and petrol cars rely on either a main or dedicated battery engine start mechanism and even if turned on, might not allow the system to be used if the driving cycle has not provided enough charge to the restart system.
