The thumbshifter can be set for two modes: friction (infinitely-adjustable) or indexed (click, click). If the thumbscrew on the top of the shifter is backed off a few turns, the cylindrical part below it can be rotated to either friction mode or indexed mode, and the setting is retained by tightening the thumbscrew finger-snug. In friction mode, the tightness of the thumbscrew may affect the stiffness of the shifter's movement as well.
Be aware that it's not easy to find cogs and derailleurs for your ride anymore. One of these days you may need to come to the Dark Side and refit it with Shimano, if you wear out your drivetrain parts.