it's do do with Brake Assist, that is if the car thinks you are in a panic stop it will bring the brakes right up to the point of ABS, as they found most people weren't braking hard enough in accident situations. now for the good bit...
How does the ML know you are panic braking??
it measures the RATE at which you stab the brake pedal
and to do this it needs a certain 'dead zone' to see how fast you are pushing the pedal... if there was no play like most cars it can't tell, (let me say I like almost zero brake pedal movement like most cars)
so it measures the time it takes for you to hit the brakes, over the distance travelled