Some trailers - especially boat trailers and rentals - use surge brakes, which need no electrical connection or controller in the tow vehicle. They activate on their own. Here is how surge brakes work.
How they work
Surge brakes use a special hitch coupler with a built-in hydraulic actuator. When you brake and the trailer pushes forward against the hitch, that surge compresses the actuator, which pushes hydraulic fluid to the trailer's brakes - see how disc brakes work. The harder the trailer pushes, the harder it brakes.
No controller needed
Because they are self-contained and hydraulic, surge brakes need no brake controller or wiring to the tow vehicle - unlike electric trailer brakes. That makes them popular for boat trailers (no electrical parts near the water) and for rental trailers towed by many different vehicles.
The trade-offs
Surge brakes are simple and vehicle-independent, but you cannot manually apply them for sway control, and they can be grabby or lag slightly. Backing up can also trigger them unless the system has a reverse lockout.
Still mind the tow vehicle
Surge brakes assist, but your tow vehicle's brakes remain critical - keep them cool and strong; see why brakes overheat towing. Fit quality pads and hardware.