An over running coupler (ORC) is basically a ratchet that attaches to your power take off (PTO) shaft and the drive shaft of your implement attaches to it. In older tractors the PTO shaft (when engaged) is directly coupled to the output side of the transmission. So while stepping on the clutch disengages the engine from the input side of the transmission, the PTO is still coupled to the output side of the transmission as are the drive wheels. If you are coasting the drive wheels will continue to spin the PTO shaft, but this rarely is a problem.
The real problem comes when you have something like a rotary cutter attached to your tractor. The spinning blade assembly will often weigh 150lbs or more. When you step on the clutch the engine stops feeding power into the transmission. However the gyroscopic momentum of the blade assembly will back-feed power into the transmission which transmits that power to the rear wheels. So you have to fight this with your brakes at the time you expect the tractor to be slowing down. It can cause you to keep going when not expected and — if you are close to something — run into it.
As long as the tractor PTO is feeding power to the implement drive shaft, the ORC will catch. After the implement drive shaft starts spinning faster than the tractor PTO, the ORC will spin freely and not transfer power back into the transmission.
An ORC is easy to install. You just slip it over the existing PTO shaft and use a pin punch to drive a roll pin through the ratchet part of the PTO and the hole in the existing PTO shaft. Make sure the roll pin is centered so it doesn't interfere with the ratcheting mechanism. It should only take about 5 minutes to install an ORC. You know it is installed correctly when you cannot pull it off, it rotates freely clockwise and catches when you try to rotate it counter-clockwise. While I assume it should be easy to remove one, it isn't something I've tried or would want to do repeatedly. Everyone I know puts it on and leaves it on. If you have a non-standard PTO shaft you can order a special ORC that also serves as a converter as well as a ORC.
I would consider this a necessary safety item if you have this type of PTO on your tractor. You run the risk of un-expected motion and running into something like a tree, which is bad enough, worse would be going over the edge of a drop off or into a ditch or pond. Items that are used with the tractor stationary, like a chipper/shredder, are not an issue as the transmission is in neutral. The only potential downside is that it does make your PTO shaft about 6 inches longer so sometimes you have to shorten your implement PTO drive shaft.
Here are a couple of pictures of an ORC installed on my 1942 Ford 2N.
Photos by Jerry Ward