I'm more inclined to belive it has two functions if its like my LCI E60 with PDC at the front (hows that for acronyms!)
When in reverse, the fronts are activated automatically, pushing the button disables both front and rear sensors.
Pushing it whilst in any other gear turns the sensors on (below a certain speed) if you want to see how close you are to the car in front and you haven't reversed in to a space.
When in reverse, the fronts are activated automatically, pushing the button disables both front and rear sensors.
Pushing it whilst in any other gear turns the sensors on (below a certain speed) if you want to see how close you are to the car in front and you haven't reversed in to a space.