Switching off the engine with the Start/Stop buttonSwitching off the engine with the Start/Stop 
button  - Starting and stopping the engine with the advanced key - Driving - Controls - Audi S5 Owner's Manual - Audi S5

Ц Stop the vehicle.
Ц Automatic gearbox: Move the selector lever to position P or N.
Ц Press the Start/Stop button ⇒ fig. 132.

Emergency Off function

Should it be necessary in an emergency, the engine can be switched off when the selector lever is in position R, D or S. To switch off the engine (max. speed 10 km/h) press and hold the Start/Stop button and keep your foot on the brake.

Х Never switch off the engine until the vehicle is stationary.
Х The brake servo and the power steering system work only when the engine is running. You need more strength to steer and brake the vehicle when the engine is switched off. This would mean a greater risk of accidents and serious injury, because you cannot steer and brake in the normal manner.

If the engine has been working hard for a long time, there is a risk of heat building up in the engine compartment after the engine has been switched off; this could cause engine damage. For this reason, you should leave the engine idling for about 2 minutes before switching it off.

After the engine is stopped the radiator fan may run on for up to 10 minutes, even if the ignition is switched off. The fan may also start running again after some time if the coolant temperature rises due to a build-up of heat, or if the engine is hot and the engine compartment is heated up further by direct sunlight.