Hydraulic trailer brakes. There is a master cyl in the front of the trailer. When the towing vehicle slows the tongue pushes on the master which in theory apples the trailer brakes.I’ll demonstrate I know nada about towing.
What are “surge brakes?”
Besides the fact that the brakes themselves are terrible you can't back up any kind of slope as backing applies the trailer brakes. The harder you push the more they come on.