When you get into the car, you set the wiper stalk at the first position up from the normal OFF position.
The sensor at the top of the windscreen measures how much water is coming into contact with the screen, and sets the wiper speed accordingly. If there is very little precipitation, the wipers may not wipe for say 2 minutes, until there is enough water on the screen to warrant a wipe.
IMO they work very well, the sensitivity can be adjusted by your dealer.