Depending on the valve used to control the direction of the cylinder you may not need this valve, and you should understand how your system works before using one. In an open center system the hydraulic pump is putting out basically a fixed flow based on the tractor rpm (basically because as the pressure builds there will be some slippage). When no valves are in use this flow just returns through the valves to the tank. Switch the valve fully and all flow will go to the cylinder, regardless of the hose size or other restrictions. The pressure the pump must create will just be more as SSdoxie said. Most directional valves will handle the transition from this flow returning to tank to going to work fairly smoothly on their own (as an example my loader valve will go pretty quickly or I can feather it well enough). The key is they still have a place for the excess flow to go. If you add this restriction device and open your direction valve fully you will just be forcing full flow a more restricted path and thus working your hydraulic system harder without actually slowing it down much. A benefit might be realized however if this restriction happens to work well with your directional valve to get the feel you were going for, but it might make it worse. If you really need to be able to tune and slow it down, the correct way is with one of the flow control valves with 3 ports with the little lever on it. This will limit the flow at any pressure and pass the extra off to the tank. My 3ph valve (no position control) seems to have detents and that transition area is hard to get. I'm thinking of adding a flow control just so my implements don't jar my little yanmar and teeth when lifting them at anything above 1800rpm or so.
