Electric trailer brake systems work using electromagnets to force the brake shoes against the brake drum to slow the vehicle. They connect to the brake light switch, so pressing the brake pedal in the towing vehicle activates the brakes on the trailer. The brake controller mounts under the dash, near the driver's knee, and includes a slide lever that also allows activation of the brakes on the trailer.
Towing a trailer increases the momentum that must stop. Applying the brakes to the towing vehicle wheels alone often does not provide enough power to stop it and the trailer. Without brakes on the towed vehicle, a quick stop can lead to a jack-knifed trailer. A better method applies the brakes of the trailer and towing vehicle at the same time. The slide lever activates the trailer brakes without applying the tow car's brake system, pulling the car to a stop through the hitch.
Proportional brake controllers use a pendulum to determine the speed at which the towing vehicle is stopping, and signal the trailer brakes so there is a smooth stop. Time delay brake controllers are less expensive and easier to install, but the time delay to activate the trailer brakes is set at a constant speed before operating the vehicle. Installing a time delay controller increases the wear on the braking system.