Does any body know how the rebound and bump stiffness works or best way to tune it
Well, basically you have oil that is being moved from one side of a piston to another, either with or without a reservoir between and you have adjustable valves on either side that control that flow. There is much to know about compression, rebound and preload as to how they work together and much of that is explained in the video below. As for adjustment, this is going to depend on your own needs and riding but basically first you will set the ride height, and that will be fully loaded with everyone and everything you most always take with you. Then find a mid position on the valves and start adjusting from there. By mid-position I mean center of the valves so close them to where they just touch and if it is between a click, open it to the first click, then count the clicks to full open, then close it to the middle. For example if there is 30 clicks to full open, go 15. Do this with both compression and rebound clickers. Then the riding starts and you will need to be in-tune with your machine to know what it's doing that needs changed. Start with the compression settings first, then move to the rebounds. This way you can focus on one side of the shock's actions at a time. Change the settings 3 clicks to feel a big difference so you know right away if that's what you want to do. Be sure to do the same to both shocks or all four shocks. If too much, back off a click or two and if the wrong direction go back to Mid position and count clicks the other way. Do know however that the front and back will be set differently because of weight and CG point.
This Elka video will help however it's s but long-winded.