Here you have a good view on how the brake is working. The brake consists of a wooden board strenghtened and holded together by irons, that go all the way around the wheel. On one side it is fixed to the construction of the outside wall by a metal hook. The other end coming down from above is connected to a large wooden beam that can be lowered or raised by the use of a thick rope going all the way down, so that it can be operated from all the different levels in the mill. By lowering the beam, the wood is spanned around the wheel so it can slowly stop. By raising the wooden beam, the wood is loosening and the wheel comes free again. In this photo, the brake is on as the beam is at his deepest point. Hope this is understandable