Scott, I think you are referring to a cross over shim. If you use a cross over the shims above it will bend over it's diameter, untill the hit the next shims in line. Now, if you remove it, the shim is now supported over a greater diameter and is therefore stiffer. Basically, leverage in increased. Think of it as a lever arm pivoting off the shim below it.
In your example of removing the 16, you would effectively be increasing the leverage, by making the 18 now bend over the shorter 14, instead of the 16 previous. If you were to remove the 12 from the bottom, this would increase the entire stack, as all would now have to bend over the 14. Make any sense? :think:
Ben, with your example, if I understand you correctly, adding a 12 crossover would make the LSC much softer, as the 24s and 22s now will be bending over a 12 (long lever arm). Once they bend far enough, they will contact the 20 and there bending will be now limited by the effect of the remaining shims (they now have to bend too in order to flow more).
I like to think of it like Marcus's analogy of a leaf spring. It helps to put it in better order in my head. Each shim in the stack must bend over the one below it. The greater the difference in diameter, the softer the spring will be (assuming thickness is equal).