In Europe, almost all trailers, caravans, boat trailers and horseboxes have nowadays been equipped with an overrun brake. It is a very simple and not very sophisticated braking system, but certainly effective.
The principle of an overrun brake is very simple. If the car slows down or brakes, the trailer, caravan, boat trailer or horsebox wants to push forward. This is called overrun. In order to prevent that the towed object going to fast for the car due to its mass, the overrun brake kicks in. There is a sliding piece mounted to the drawbar near the coupling. This contracts as soon as the towed object tries to overrun the car. As a result, the brake cables are pulled and the towed vehicle brakes. The result is that the trailer, caravan, boat trailer or horsebox slows down. Increased pushing force results in increased braking force. If the car stops braking, the brakes of the object release and you continue towing. With a properly attuned overrun brake you should not notice the braking movement of the towed object during driving. However, always try to prevent sudden braking as much as possible and increase the brake pressure gradually.
An overrun dampener (shock absorber) has been installed in order to prevent that the overrun brake immediately starts working if you release the throttle or brake slightly. There are various dampeners available, depending on the mass of your caravan. If the trailer, caravan, boat trailer or horsebox is heavier, the dampening effect has to be bigger as well. You ensure this is working properly you should ensure the vehicle is check by a qualified technician of the manufacturer.
If you are driving in reverse, the trailer, caravan, boat trailer or horsebox is pushing against the car as well. In order to prevent the overrun brake from taking effect, a so-called reverse mechanism has been included in the brake drums. This system also ensures that if you pull the handbrake of the towed object, the caravan can still move up to half a meter backwards.
Descending in the mountains
An overrun brake has its limits on steep hills as well. If you are descending down a steep slope with a constant speed (by braking with the engine), the caravan will constantly push against the car and thereby activate the brakes. This could result in the caravan brakes overheating. The best thing you can do to prevent this is regular breaks to cool down the brakes.