Master cylinder, hydraulic system (brakes),cost prohibitive unless you do it yourself.Basically, your tow vehicle slows down,tongue has hydraulic actuator, pushes fluid against the calipers.Boat trailers,U-haul trailers have them,just look.
IMHO I'd NEVER have a trailer with electric brakes thats being used in a timber setting.Bad things happen to wires even if they're protected.
NOW,that being said,people thinking it's ok to pull overloaded with tractor, ATV,UTV (on your private property ),I don't need or want to get into public roadways,IT'S ILLEGAL,that think they should really worry what happens if you pull a hill and load is trying to pull you backwards.
Lost a friend 15 years ago.
Hauling corn from his field to the grain bin .Don't remember what tractor he had,but pulled a gravity flow grain cart.HEAVY,field road getting back to bin
Came to a hill,halfway up tractor started to loose traction, spinning, tractor couldn't hold the loaded cart,rolled backwards,jack-knifed,killed him.
2nd week into his retirement ...
Be careful with heavy loads