There is a device that has two moving discs, with a stationary, fixed disc in between them. The outer discs rotate when the dw is moving. The outer discs have long bolts that hold them together, and these bolts are perpendicular to the discs, when the rope is being pulled. The discs are allowed to spin around the fixed disc. But when the pull rope is released, the cab wants to drive the discs, instead of the rope. When this happens, the discs twist in opposite directions. This makes the bolts tilt, and as they tilt, the distance between the moving discs is reduced, because the bolts are tilted. This makes the outer discs come closer together, squeezing the fixed disc, and thusly braking the unit to a stop. Pulling on the rope twists the outer discs in the other direction, straightening out the bolts, allowing the moveable discs to once again move father apart from each other, allowing the cab to move.
