Taken from another forum (but worded better than I would have, lol)
"At slow shaft speed a larger hole thicker fluid will feel exactly the same as the equivalent small hole light oil settup. However, at faster shaft speeds, like encountered from jump landings and sharp edges on the track, the smaller hole pistons will "pack" and become stiffer than the larger holed equivalent. It has to do with fluid dynamics.
It's the equivalent to having a slow speed, and high speed compression adjustment on full size offroad shocks. The number and size of the holes in the piston is a rudimentary way to adjust the two independently in RC shocks"
Also an interesting (if involving) read:
http://users.telenet.be/elvo/