I had one in a 1949 Rover 60 P3. It was activated by a Bowden cable. It was lethal descending hills with no engine braking and lousy brakes. Saab adopted it for a different reason; no torque reaction when lifting off on ice and snow means much less likelihood of a skid.

VAG has introduced a freewheel to a new DSG, but freewheels will not now save fuel with manuals because a modern engine shuts off its fuel supply entirely on deceleration, or if you lift off while descending a hill. The engine is turned by the powertrain. An idling engine (which it would have to be while freewheeling) uses some fuel.