Surge brakes most simple. The harder you brake, the more the trailer (dolly) pushes and brakes itself harder. Backing up a grade may cause the surge to brake since the weight will be pushing against the cylinder that activates the brakes. No controller like electic brakes
Electric brakes don't have that problem. Light or no load on a trailer (dolly) can cause the brakes to just stop the wheels from turning because there is little or no weight on it and no resistance to the braking effect. The controller must be adjusted to match the load you are trying to stop.
