This type of trailer coupling activates the mechanical brakes on trailers via override principle.Brakes are a legal requirement on trailers and caravans with a GTM of more than 750kg. As the driver breaks the tow vehicle, the inertial force of the trailer against the coupling compresses a spring within the coupling. This force triggers the brakes. Brake-override couplings generally include a handbrake to lock the brakes in place when the trailer is unhitched from the towing vehicle.
Override brake coupling normally suited to a 50mm Australian Standard tow ball, where the brake system requires activation from a sliding or overrun coupling.