The unit modulates your brake light display. It flashes the brake lights when you apply the brakes.
It's a safety feature. I've installed them on all the bikes I've had since, well, since about 1982.
I haven't been rear ended yet, but then having a little luck never hurts, either. I got proactive about this when I got to watch my wife who was riding our Yamadog 650 home get hit from behind by a cager. Not pretty.
Anyway, I like this unit. It does everything I need. I believe it can even be programmed for a different mode if you don't like the stock factory setup.
And it's about 1/10 the size of the unit from 1982!
"First You Fly, Then You Die!"--Los Lobos