The box in your cab controls the amount the brakes are applied, proportional to what you are applying and the setting you have on the controller. The breakaway device has to have it's own battery that will activate the brakes if everything else disconnects from the tow vehicle.
2015 Legacy SR340 360RB
2014 Jeep Wrangler Unlimited Toad
Gigi, Poppy and Sadie Lady...On the road, but not full time!