That's the older design trailer brake controller that incorporates a hydraulic connection to the master cylinder. It will have a power wire connected to battery voltage, an output line going to the trailer brakes, and a smaller-gauge third wire connected to the brake lights. It works with either hydraulic pressure from the brakes, or manually by moving the handle side to side. The knob turns to adjust the sensitivity, so the output to the trailer can be reduced when running unloaded to avoid locking up the trailer brakes. Later model electronic controls don't have the hydraulic connection because it interferes with anti-lock brake systems.
Jerry
