The automatic tensioner that comes with the bike is essentially:
- a coil spring that turns a screw
- the screw extends a piston
- the piston pushes the chain guide against the chain
The spring gets weak after a while and does not apply enough pressure. When it starts to go bad, you get a light buzzing sound just above 5,000 rpm. As it gets worse, the buzz gets louder and over a wider rpm range. When it gets really bad, it sounds like your bike is about to fall apart.
The manual tensioner is just a bolt that presses on the chain guide. You tighten the bolt every 15,000 km or so. There is a lock nut to hold it in place.