The overrun coupling is part of the braking system on many braked trailers. Correct drawtube movement and linkage operation matter because the coupling has to transmit braking demand to the wheel brakes and then release cleanly.
This workshop image shows drawtube work in progress. Coupling faults are diagnosed together with the brake linkage and wheel-brake condition rather than treating each part in isolation.
These images are genuine workshop photographs. The exact parts and repair approach vary by trailer, so this case study should not be treated as a fixed repair specification for another trailer.