In addition to wearing the proper safety gear while operating a chainsaw, keeping the chain properly tensioned can help lessen your chances of injury. While you should consult the manual that came with your chainsaw for specifics, there are tests you should perform before you start cutting to ensure your chainsaw chain slack is properly adjusted. In addition to lowering the chances of getting hurt when the chain comes off the bar, keeping the chain tightened within specification helps the chainsaw cut more efficiently, and reduces wear on the chain and other components. Generally, a chain saw's chain should be tight enough to prevent sagging away from the guide bar, but still allow the user to pull it away from the bar with finger tip pressure. If your chainsaw didn't come with a scrench, or you lost the original, you can always pick one up through Amazon for just a few bucks.