Your lever should be spring loaded to neutral. The more you move the lever away from neutral the greater the hydraulic flow. The more flow, the faster the cylinder will move. So you can control the speed by how far you move the lever.
Some valves will have a detent that the lever will stay in position when in detent. That is until the cylinder reaches system pressure then the lever will "kick" out of detent. With detent, you move the lever into detent, let go of it and (depending on direction moved) your grapple will open or close, and lever will kick out (grapple will remain open or grapple will remain closed). My valve has detent and that's how I use it.
Your valve could also have float. Usually in only one direction of the lever. If you push the lever too far the valve enters float. Not useful with a grapple. Sometimes I accidentally move the valve to float and the grapple stops moving. Just pull the lever out of float.