The way the bristles make contact with the brake disc is very opportunistic, to say the least.
You will notice, as you say, that if you slam the brakes hard on the highway the light will go off (when it started just flickering). I don't know why it does this (maybe to much friction cannot ground the wires properly).
Other times, when the brakes are wet, the wires become more sensible (due to the water making ground) and the brake pad light will come on more often, or stay on constantly. Once the brakes are dry, the attitude changes.
Anyway, it is interesting to see how simple and logic the sensor systems is layed out. It makes you wonder why not everyone else installs brake pad wear sensors like these.
If you change your brake pads once the light starts flickering, you still have good life left on your pads, according to the local M-B dealer. Once you first see the brake pad light go on, you usually still have about 1,000 kms. of life on the pads (it all depends on your driving habits and terrain conditions).