As far as I'm aware it’s just on all the time. Well, it certainly was when I was cutting lanes the other day into the left and the car thought I was too close to the van in the middle lane (which I wasn't ) and it slammed on the brakes for me.....fortunately only for a second - but it proves it works!
Haven’t had a slam on the brakes yet, but had some alerts, when something big (like a bus) slowly passes me, or when I slowly pass something big, maybe approaching a roundabout etc.
There’s also been a few times when I have been quite close to the car in front (not BMW/Audi sales-rep close!) and the cautionary sign you’re closer than the system would like to you be hasn’t signalled at all, yet another time, not even as close and it has signalled. Sometimes I attribute it to maybe the car in front dipping down a hill just before me, so in effect the radar is looking over the top of their car. Other times I wonder why the car isn’t indicating that I’m closer than it would like.
Sometimes it seems more concerned with a change in speed rather than a change in the gap size e.g. very close to the car in front, but it’s accelerating away from me slowly and opening the gap – system not concerned. Huge gap between me and the car in front and the car in front brakes a little – car becomes concerned even though the gap is still huge at the point of alert.
I could see it being an issue when you are maybe stuck behind something and you get quite close to what’s trundling in front of you, ready to pounce on a gap in oncoming traffic and get past, then the car bangs on the brakes mid manoeuvre to come out. Hasn’t happened to me yet.