Pre sense front works at all speeds - regardless of whether ACC is on or off. And assuming you've switched it on in the MMI.
It's just the complete autonomous braking to a complete stop that works only under 19 mph.
At all other speeds, it will give you the BEEP, Visual Warning, and Braking JOLT, then only apply partial (up to 30%) braking pressure.
Then when you wake up and press the brake pedal, it will add the remaining brake pressure required to stop you before you hit the car in front, but allowing for max distance for the following car.
edit: This is quite a good explanation:
cartechnical | Audi safety track test - YouTube