...It also has hydraulic surge brakes. How do those work?...
The part built into the coupler is a hydraulic master cylinder, just like the one in your car. Instead of pushing on it with a foot pedal, the tow vehicle pushes it by resisting the forward motion of the trailer. Brake fluid transmits the force to components at each wheel which are just like the ones in a car (in this case, one with drum brakes).
If the tug brakes
harder, the coupler is pushed with more force, and the trailer brakes
more. It is a proportional braking system which requires no electrical
power or connection.
The breakway feature is a cable hooked to the hitch so that if the trailer comes off (ball or coupler fails) the cable pulls the brakes on. From Jim's comment, I assume that this feature was not originally included in the U-Hauls.