Well I replaced my Rear shock with a 95 gsxr750 shock about 3weeks ago and I had some trouble adjusting it (bouncing all over the place)  So I fixed that by adding preload and going highest dampning (it was extremely low-When I sat down it sank alot)  Anyway Now after riding for awhile and getting used to the shock I can get the rear to spin when I hit bumps.   So I know my rear is leaving the ground. Kinda fun, kinda not fun.   But what would make the tire leave the ground?  Like taking a corner ~25mph and hitting a wrinkle in the pavement and the rear will spin (RPMs go up but speed doesn't then it catches hehe)  THanks

sounds like rebound is too harsh...wheel not coming back down fast enough. that'd be the button/dail at bottom of the shock. Did you do anything to that to begin with?

Also read through my site's page on suspension setup-it is written specifically for the gsxr shock.

also the preload might be a bit high also
when you sit down on the bike the rear should drop about 20-25mm....

i have the same problem with teh katana shock..

I have some sort of round disc that should rotate on top, and the preload slots on bottom..

which do I adjust?

You may need more preload or less rebound...whichever has room to go. Essentially what happens is the spring compresses and as it tries to expand the fluid column is too restrictive to let it expand quick enough to maintain road contact. Free up fluid by lowering rebound or strengthen the spring and reduce its compression a little by making it stiffer - raising preload.
Cool.
Srinath.