The following was a response to a previous post and I hope will be of some assistance;
OK, let me give this a try. I'm not even close to an expert (but am learning). First the shock and fork are slightly different, but the principals are similar.
Picture a piston on the end of a shaft, now that shaft is in cylinder filled with fluid. The piston has holes in it that pass oil from above the piston to below and below to above as the piston moves through the fluid (if it didn't there would be a hydraulic lock and the shaft could not move).
Now the damping effect is created by the resistance of the fluid moving through these holes in the piston. The smaller the holes, the greater the resistance. Similar to passing a cupped hand through water or an open hand. In a crude system this is what you've got to control damping.
For our purposes though, we need to better control that fluid flow to give greater control of movement. Shims (thin little washers) are stacked on the top and bottom of the piston, covering the holes, or ports.
Now you must imagine this piston having 8 holes. The shims on the top cover 4 of these holes completely, the remaining 4 holes are covered by the shims on the bottom of the piston, but exposing the other 4. So, on each side there are 4 holes covered and 4 holes exposed.
Now move the piston upwards (as if the shock is compressing). The oil is displaced from above the piston and forced to flow to below. The shims are restricting this movement, but because they are thin, they deflect allowing the fluid to pass by.
On rebound, you will see the same effect takes place only in reverse, wit fluid travelling through the other set of 4 holes, past the shims and into the upper side.
This piston and shim arrangement is the "active" valving that you will hear referred to in this forum. Active in that it is moving through the oil, as opposed to being stationary and having oil flow through it.
...starting to make any sense?
Now, I going to throw another piece of the puzzle your way. Go back to when we compressed the shock. As the piston is moving upwards through the oil, the shaft that is pushing it is also filling the lower chamber of the cylinder. If the cylinder is sealed, then you again have a hydraulic lock situation as the internal area that the fluid occupies has now been diminished by the additional area of the shaft. (like putting an ice cube in your drink, that level in the glass rises).
So we have to do something with this displaced fluid. A second valve is provided, in a similar fashion to the one I just described above, only this one is stationary (ie: passive). This displaced fluid is passed through this valve arrangement and controlled in a similar manner (with stacked shims).
I hope I haven't made this any more complex than it has to be, but here's another curve...
In addition to the fluid that is passing through these valve assemblies, builders have given the oil an additional "out". Bypass holes or bleed are usually provided to allow a percentage of the fluid flow to bypass this valving arrangement. For simplification (if that word can even be used at this time ) I will not get into different types of bleed too in depth.
Remember our piston on the shaft? Make the shaft hollow and provide holes in shaft, above and below the piston so that some oil can flow past valve freely. Now we install a restriction between those two holes and into that restriction we install a tapered needle that can be threaded into and out of the restriction hole, thereby allowing us to control how much fluid travels through our "bypass". Voila! this is your clicker! (rebound that is anyway).
Do a similar bypass in your "passive" valve. You now have your compression clicker.
I am sorry for the length of this post, but I hope you understand my ramblings. If anyone has any additional items to add, or corrections to make, please do.
DBD, if you ever get the opportunity to see inside a shock or forks, take a close look and try to trace the flow of the fluid. It makes much more sense when you can see it in your hands.
Hope this helps!