Essentially a Ball and Chain - minus the chain. This device was primarily intended to be used for prisoner transportation since it is relatively unobtrusive and designed to slow a prisoner down and prevent escape.
The large steel donut is locked around the leg, above the ankle, using two screw bolts. The inside diameter is such that it cannot be removed over the ankle. The frame which is an integral part of the boot (and screwed into the heel) keeps the donut elevated above the ankle in order to prevent damage to it.
It is possible to walk while wearing one of these, but running would be difficult if not impossible due to the heavy weight moving around at the end of the leg.
Oregon Boots were so named because their inventors tended to live in Oregon. It is not clear why this is the case, just that it is.