That is an auto tensioner. You'll need to unscrew the whole tensioner, loosen the small bolt in the side, screw the tension rod all the way in and hold it while you tighten the small bolt to hold it in place. Next, put your chain all on. Then screw the auto tensioner back in. Then, loosen the small bolt and the tensioner should come out to tighten the chain. Tighten the small bolt back up and close up your chain case.