Preload is the amount of initial tension on the spring(In this case an elastomer.)
The knob, when turned clockwise, pushes down on the elastomer spring and makes the fork "stiffer"
I haven't done a Jett In about two years, but first remove the top caps and pull the elastomer out, then if there is an allen screw at the base of each leg, you loosen it most of the way, tap it with a hammer to break the internal leg loose, finish unthreading the screw, and slip it apart. If there is no external screw, look down inside the fork, there will be a 5 or 6mm allen down there. Remove it and the legs should slip off with a little persuasion. Wipe everything clean, put some fresh grease on the internal bushings and slip it back together.
This fork is getting hard to get parts for, but there may still be some stiffer spring kits if cranking the preload all the way does not work.
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I am in the woods and I have gone crazy.