tut request: Zmask

I must be thick on this. I’ve read the beautiful page in wiki on Zmask and I just don’t understand it. What IS Zmask, do I use it with multiple render layers. If I click on it, it seems to stay on through all the render layers/ scenes!! That can’t be right. WHen I render the Zmask, what layers do I select? IS there a render output of Zmask, if so where is it and how do I use it.?

Any tuts out there that go a bit slower explaining Zmask?

This all comes from trying to use edge render and hair. Turns out that edge render writes over/ shows through/ hair. If I try Zcombine in compositor it still shows through, and alpha over doesn’t take into account object order from the camera so next idea = Zmask, sigh.

If anyone cares, here is a thread where I worked it out.
BTW using AllZ in the render settings does the same thing for every layer but increases render time.