Can someone explain how this cam chain tensioner works

It is off my 82 KZ305. When it is off the engine, with about 12 lbs of force, I can push the shaft all the way into the housing and lock it there with the small bolt at the end of the housing. How ever, once installed in the cylinder block and the shaft is released, it will go out the required amount and can not be pushed back in. I know most tensioners have some kind of ratchet mechanism  that will not allow chain tension to be released, but I see nothing like that on this one. What is keeping the plunger from being pushed back in? 

 
 

looks like purely spring tension (not uncommon in the car world)

It's the ball bearing collar that stops the shaft from retracting.
Combined with it's steel housing (which has a slight taper) it functions in a similar - ish way to a starter clutch in that it only provides grip in one direction. 

The style of tensioner your showing is the retainer type and by far the worst tensioner ever used. They were a brainstorm of someone and started being used in the late 1970's on several of the Kawasaki's'. As 1075 mentioned that cage of bearings "wedges" the shaft in place because of the slight ramp in the housing of tensioner body. When checking and before installation understand the concept first!! The rod should ONLY pass one way thru easy then lock going the other direction. Look for flaws and dimples in both shaft and race pressed into housing. Like Zed said, same principle as a sprag starter clutch used on a lot of these Kawasaki's'. Difference is though a starter clutch fails it won't start, tensioner fails and you'll be buying a box of engine parts.

Unfortunatly, the factory 82 kz305 service manual is just a supplement to the kz250 manual. The manual details how to install and adjust the manual tensioner on the KZ250, but when you get to the KZ305 supplement, it only states the adjuster is automatic and no adjustment is needed.
Well, a few months ago, I refreshed the top end of the KZ305, and thinking the adjuster was just a spring loaded shaft with no one-way mechanism, I must of had the timing chain way over tightened.
100 yards into the test ride, the timing chain snapped. That box of engine parts asphalt900 mentioned should arrive sometime next week.
I'm assuming the correct procedure is to hold the plunger back using the bolt on the end of housing, Install the tensioner, then back out the bolt until the plunger is released and snaps forward.