Please note the following points to prevent the Audi A6 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.Depending on the weight of the Audi A6 and trailer and the steepness of the slope, there may be a tendency to roll back downhill when driving away from a standstill.