Info taken from the TW Wiki Site
Rear shock adjustment. The stock rear shock affords you 3 pre-load settings. The bottom 'disc' which the shock spring sits upon is held at one of three height positions by a springwire clip. This wire locks into one of three grooves near the shocks base and fixes the bottom disc in that position. The wire is concealed under the lip on the base disc, but you should be able to see the shallow lower groove at the very bottom of the disc.
I believe the factory setting for this springclip is the middle groove. This being the case, if need be you can go to a 'stiffer' setting by repositioning the springclip the one groove highter. Now you will have a shock with a tighter spring and a firmer (rear) ride, but since the spring is compressed more, the bike (rear) will sit slightly lower.
Should you move the springclip to the bottom groove, you reduce the tension on the shock spring and make for a softer ride. A secondary result of a soft adjustment is a relaxed/longer spring (JVT). Since this will raise the rear abit, it may create a new problem for a light person that is just barely able to touch the ground. The above are the factory adjustments.
Should you want to see, and adjust the spring setting on the rear shock, you will want some sort of device/tool to pull 'up' and hold the tension of the shock spring while you remove the bottom springclip and base plate. Springs are under a great deal of tension; USE EXTREME CAUTION AS SERIOUS INJURY IS A REAL POSSIBILITY.
Here is a picture of the preload clip posted by (davedyno) on the old TW site. The clip grooves are about 1/2" apart