Some are just a point that protrudes down into a cone below the threads, on others(newer ones I think), the point is pinned in place and can slide up and down a little. When lifting the 3PH, the fluid comes up from the bottom of the cone and flows around the tip to reach the cylinder. When lowering the fluid must flow down and around the point to exit out the bottom of the cone. On the ones with the sliding tip What will happen is if it is set too far closed, it acts like a check valve, fluid comming into the cylinder when lifting, lifts it up and out of the way, but when you try and lower the 3PH, the fluid flow/pressure forces the tip down into the cone, sealing it. I think they did this so it could lift at full speed, but only lower at a reduced speed to help keep the operator from slamming heavy impliments into the ground accidently.
If that knob is closed to the point where it is acting like a checkvalve, The hydraulic pressure from the cylinder supporting the weight of an impliment will LOCK that knob in place, at least it does on mine. Can't hardly turn it with a wrench if it is too far closed with an impliment in the air. Jack up the impliment, or back it up and onto a dirt bank to completely remove the load from the hitch, and I can then unscrew that valve handle with 2 fingers... The handle should be held onto the square shaft with a drift pin. the handle is pretty fragile... As mentioned, there is a setscrew down in front that must be unscrewed to completely remove the valve for cleaning.