RE: Official Smartech Thread
they replace the hex mount for the wheel. They slide over the outdrives and have a set screw that goes through them were the little metal pin held the original hex on. then instead of using a nut to hold on the wheel the end is threaded and you actually use a screw.
The clutch bell replacement is so it will last longer more then anything. The stock bell i got with mine was cast aluminum which doesnt last long at all.