The train was screw-coupled; each carriage was first coupled by a loose chain and hook coupling to the next; the slack on this was then taken up by a turnbuckle screw arrangement, until the buffers of the two carriages were touching. To uncouple, there needed to be some slack in the coupling; as the train had stopped all the couplings were under tension. Once the vacuum brake connection to the rear portion was broken, any attempt to introduce slack into the coupling between the two portions would be defeated by the rear portion settling back to rest its weight upon the rear van brakes. To assist uncoupling the front van guard therefore scotched one of the wheels of the sixth vehicle, that is, the front vehicle of the rear portion being detached. Loosening the turnbuckle thus transferred the weight of the rear portion to the scotch on the sixth vehicle, rather than to the rear van brakes. The couplings to the rear of the sixth vehicle remained under tension, and the slack introduced remained in the coupling between the fifth and sixth vehicles, which could be unhooked.