Surge brakes? Don't they work mechanically? Don't you put a pin in the tongue to keep from applying the brakes?
ChuChu, you would be correct for older surge brake systems with drum brakes. The drum brakes actually were so inefficient, you could back up anyway, even if you didn't bother putting the lock-out in in place on the tongue's actuator.
Modern bass boat trailers almost always have disc brakes. They are so efficient that they will bind up, lock-up and actually damage the brake system if the pressure is allowed to build. Manually pinning the actuator every single time would be pain overlooked by some, causing damage. The solution is an electrical solenoid that opens a valve for directing fluid back to the master cylinder. This way, the brakes are disabled every time the trailer is backed up. That solenoid is powered by the Reverse wire from the tow vehicle. This is why modern trailers have 5 wire lead connections instead of 4. Putting the vehicle in reverse always disables the surge brakes in this manner.
The surge-actuated brakes are still mechanical - it's only the disable feature for backing up that's electrical.