Hydraulic surge brakes are most popular on boat and watercraft trailers. Sealed hydraulic systems deal better with immersion than electricity, and the normal trailer weight is determined more by the boat on it than by how much you load in it. This brake system is self-contained on the trailer, consisting of a master cylinder on the coupler and brake lines and wheel units much like on your truck.
When a tow vehicle slows down, the trailer wants to push against it, and as the trailer pushes against the coupler, a lever inside it exerts pressure on the master cylinder just as pushing the brake pedal in your truck does. As the cylinder is pushed upon, hydraulic fluid is pressurized and goes through the lines, pushing the calipers or shoes against the discs or drums, thereby slowing the trailer. The more the tow vehicle slows, the more pressure is exerted on the trailer brakes, and many brake systems are sensitive enough to engage when you downshift or engage the exhaust brake on a diesel engine.
Electric trailer brakes require a controller such as this new Tekonsha Prodigy. The self-adjusting Prodigy has a "boost" function for initial heavy-brake application and is the only interia-based controller that works proportionately in reverse.
Surge brakes historically have used drum brakes, but disc brakes are becoming popular because of benefits like heat and water-shedding ability and no adjustment required for wear. Surge brakes are usually set up right from delivery with the boat and require little to no adjustment for braking force. However, they may require adjustment to the shoes for wear, need bleeding - especially when parked for long periods of time, and the coupler must be kept lubricated so it slides easily under load. When surge brakes become troublesome, the culprit is often low fluid level or a bent or pinched brake line.
Since surge brakes engage when the trailer is pushing on the tow vehicle, they also tend to engage when backing up. Often a minimal, smooth throttle input will get past that, although many new trailers come with a fifth wire that will release the brake pressure for backing under load. For long descents, surge brakes should be treated like truck brakes, using regular short applications of moderate braking with intervals off for cooling them.
Except for the reverse-release signal wire, which can be triggered by back-up lights, there's no wiring for hydraulic surge brakes. However, you'll still have to wire the trailer lights.