It is a shim to up the preload on the spring. It also does a little on a final. It's mostly used to get your engagement where you want it. Why are you putting a "stiffer" spring in. If your over reving and that Bender spring is more than a year or so old, then you need to just buy a new Bender spring for that kit. The springs will sag over a year or two of use and you may be under reving now. I would not just add a stiffer spring or you will slow your sled down. You will not upshift as fast and you might just end up over reving. Clutch it to your target RPM, if your over go with a softer finish spring or more weight in the weights. If your under you need a "NEW" spring or stiffer finish spring or lighter weights. Doug