Although there are several different styles, we can divide brake controllers into two distinct groups -- proportional brake controllers and time delayed brake controllers.
Proportional brake controllers use a motion-sensing device to detect how fast the tow vehicle is stopping. The moment the driver applies the brakes, the brake controller applies the same amount of braking power to the trailer's brakes -- if the truck is stopping quickly, the trailer will stop quickly; if the truck stops slowly, the trailer will stop slowly. In a situation that requires heavy braking, for instance, a proportional brake controller will cause the trailer to stop at exactly the same time as the truck does. This type of brake controller provides the smoothest braking, and because both systems are doing the same amount of work, it reduces the amount of wear on each vehicle's braking system.
Proportional brake controllers are also known as pendulum brake controllers because of the way they sense motion. These devices use the position of a pendulum as a motion-sensing device, and drivers typically need to calibrate them before using them. When the vehicle is on a level plane and the pendulum is pointing straight down to the ground, the brake controller doesn't sense any motion and won't send any signals to the trailer's brakes. When the vehicle moves, however, the pendulum points toward the rear of the vehicle. As soon as the vehicle brakes, the pendulum swings forward. Depending on how far the pendulum swings, the brake controller sends a degree of power to the trailer's brakes.
Time delayed brake controllers, on the other hand, provide a pre-determined amount of power to the trailer's brakes when the truck stops. The power is set beforehand by the driver and depends on how much trailer weight he's towing. A delay will always occur when the brakes are pressed; however, a sync switch allows the driver to adjust the length of the delay. Time delayed brake controllers put more wear on braking systems, but they're less expensive and easier to install than proportional brake controllers.