The tensioner is a spring loaded plunger. It has a one way "lock" so it can go out but not in. Each time you hear it "Click" that is one step in the "Lock". It is designed to take up the slack from normal wear (Chain stretch, wear on plastic cam chain guides... )
I'm resurrecting this zombie thread after linking it from SacMike's "Tales of Mechanical Woe".
In regards to what Rogue wrote above, should anyone ever read this thing, the cam chain tensioner IS a spring loaded plunger, BUT there is no "click" and no "lock" when the adjuster actually makes any adjustment. The spring, which is wound around the tensioner shaft, forces the shaft, which is a large screw, out through torsional tension. The adjuster simply screws out when there is slack in the chain. Theoretically, since the shaft is a screw, and the spring forces the screw deeper into the motor during slack-reduction, it can't retract.
But in truth, that theory is false. When the spring reaches the end of its effectivenes, i.e., no longer "pushes" the adjuster shaft out, there is no longer any spring tension to prevent the screw from backing itself out, causing even more slack in the cam chain.
Don't ask me how I know.