Please note the following points to prevent the Audi A8 from rolling back unintentionally on a gradient.
Pull and hold the switch 
and press the accelerator. The parking brake will remain engaged and prevent any tendency to roll back down the slope.
and press the accelerator. The parking brake will remain engaged and prevent any tendency to roll back down the slope.You can release the switch as soon as the engine is delivering enough power to the wheels.
as soon as the engine is delivering enough power to the wheels.