OVERRUN BRAKES WORK AS FOLLOWS
An overrun brake (or surge brake) is a German invention that have been used all over the world for over two decades.
This is how it works: The coupler of the vehicle being towed is attached to a dampened sliding mechanism (A).
When you step on the brakes, the towing vehicle slows down while the vehicle being towed continues to move forward, thus exerting pressure on the sliding mechanism. Attached to the rear end of the slider is a brake lever (B) that rotates on a pivot.
When the sliding mechanism pushes the upper part of the lever rearward, the bottom end simultaneously moves forward.
Attached to the lower end of the lever are two cables (C) or rods that lead to the drum brakes on the wheels. So whenever the lower end of the lever moves forward, the cables are pulled to engage the brakes of your trailer or caravan.
Whenever a towing vehicle accelerates the sliding mechanism is once again extended, the brake lever moves forward while its bottom end moves backwards which in turn releases the brakes.