Only works if you have LED headlights. I did notice it on MK3 pre-facelift ones...but only when I dropped speed, not when I went past 68mph.
I believe the lights would raise about 3 or 5%. I only noticed it, because I suddenly had to stop quickly from a fast speed.
On the manual it states that, the beam would lower back to normal levels after you have gone under a certain speed for 30 or 60 secs (cant remember which). When I stopped, noticed the beam lowered a little...I thought it was autolevelling (like when you start) but no, it just went down a fraction, and never came up again.