Not sure on the yz cdi but the shift star mod is when they cut or round out the shift star for smoother shifting.It takes the notchiness out of the shifting.Greatest $20 mod you can do.Really improves the smoothness of shifting.Flotek does them for $20.If it is an 03 or newer the shift star is welded to the shift drum.If it is 88-02 you can take it out by removing the clutch cover and remove the star by the torque screw.Hope this helps