Trailer brake controller ?

What does the red stop light wire do on a trailer brake controller? I helped wire in a brake controller on a buddy's Toyota pickup and it workes fine but I'm wondering exactly what the stop light wire does hooked into the cold side of the brake pedal wiring.

Does it sense the brakes being used and turn on the controller? Seems like the pendulum deal inside the controller does this. Or does the wire just turn on the brake lights on the trailer? Just trying to understand how things work.