An overrunning PTO clutch is a device that allows a tractor's power take-off (PTO) shaft to be driven in one direction, but to spin freely (freewheel) in the other direction. The ratchet mechanism in an overrunning clutch is similar to the freewheel hub on a bicycle. The frewheel action of the overrunning clutch prevent equipment from "driving" the tractor through its PTO shaft.
A typical example is a rotary mower (sometimes called a "bush hog") that is being operated behind a tractor with a transmission PTO. When the operator pushes the clutch to slow the tractor, the inertia of the spinning mower blades put a force onto the PTO shaft. This force drives the tractor forward, causing an unsafe situation.
Some tractors with transmission-linked PTO shafts have an internal overrunning PTO clutch provided by the manufacturer. For those tractors that lack the cluch, there are a number of companies that built after-market overrunning clutch mechanisms. A typical device is a quick-coupler that snaps onto tractor's PTO shaft, between the tractor and the equipment.