Hi
the switch can actually trigger a relais which in turn triggers a horn of some sort. The Switch can only give you about 100mA at full Battery Vcc so a relay have to enable the real juice. I use the switch as additional brake lever.
In my (defective controller... 3 times now in about 30 month) cases I never used the function as motor was dead after each (1=accelerate with no throttle, 2=no reaction anymore, 3=sometimes it works sometimes it doesn't) failure (but I've two MP3s and one is in constant usage). Currently I've an open ticket again, as the new controller seems to have a problem with regenerative braking and after the current issue I guess I switch for convinience to an external controller with a bit more reliability :-(
Alan from GM Ca is surely able to tell you more about it.
Cheers
Sam