The controller is in standby mode until the brake-lamp switch is activated by pressing lightly on the brake pedal (most vehicles illuminate the brake lights before the brakes actually start working). Roughly a tenth of a second after that lamp circuit is energized, the controller sends a variable amount of voltage to the trailer-brake solenoids, which convert it to mechanical energy so the brakes can convert the motion of the trailer (kinetic energy) into heat. It's important to know the brake-lamp circuit is the trigger for electric trailer brakes for two reasons: One, if the light switch or its wiring is bad, there will be no automatic trailer braking; and two, as long as your foot is on the brake pedal the trailer brakes are activated (you're wasting voltage sitting at a traffic light on flat ground).
While routine maintenance of electric trailer brakes is less involved than hydraulic surge brakes, the electric units require more investment in wiring and time. And the simplest part of that, for any trailer, is a good ground connection; the tow ball is not an acceptable substitute.
This Four Winns trailer uses...
This Four Winns trailer uses a typical surge-brake coupler that pushes more fluid to the brakes based on how much it's "squeezed" between truck and trailer during braking. It's self-contained but must be kept lubricated and full of fluid for best operation.
Trailer brakes add only one wire to those required for the lights and trailer-battery charge. The controller usually needs four wires: 12VDC +/-, the signal wire from the brake-lamp switch, and an output to the trailer. Many new trucks come with a dedicated four-pin connector under the dash specifically for brake controllers, while others include it as part of a towing package. Truck/trailer connectors themselves have yet to be standardized, though most RVs use a seven-pin plug and most vehicle manufacturers include one as part of a towing package. Given the increasing complexity of modern truck electrics and the fact that the majority come with separate turn and brake-lamp circuits (while trailers are still primarily a combination turn/brake), we'd opt to pay the nominal sum and let the factory build in the tow wiring from day one.
Regardless of design, trailer brakes should be checked prior to every trip. This requires a short drive instead of a visual inspection. Trailer brakes are subject to changes in temperature, humidity, and wear just like any friction device. Trailer brakes should never be considered an excuse to further overload a tow vehicle or a trailer.
All trailers should also consider a break-away device (again, requirements vary by state). On hydraulic surge systems, the break-away is usually a chain attached the tow vehicle. When the chain is pulled taught, as when a hitch failed, the chain pulls on the brake cylinder, applying heavy braking force and stopping the trailer. On electric brake systems, the break-away is powered by a smaller, secondary battery on the trailer itself that will engage the trailer brakes if it disconnects from the trailer.
Sanely driven and properly maintained, trailers tend to stay behind the tow vehicle, but both Murphy and Newton suggest that an object in motion tends to keep going. If anything in your tow hitch fails and the trailer becomes disconnected from the truck, a break-away will keep it in the mirrors and not the side windows.