The owner's manual explains it.
I adjust it if I feel excessive lash while rolling off the throttle. You want about 1.5" to 2" of slack along the bottom run of the drive chain. Do not make it too tight.
You also need to keep the primary chain adjusted, I believe with about 0.5" of play. But you should look it up in the manual.
I don't have my owner's manual or Snidal with me right now, so I can't check myself.
Jeff