Like you guessed, it is easy and you'll find that you use it alot - basicallly, anytime you want to mask something, but not have the mask move with the layer...
First, make sure you have mattes column expanded on timeline (little box on bottom left hides/shows this). Place the solid directly above the layer you want to matte (if you have a lot of layers, you could precomp this).
Then, on your footage layer (below the solid), set the matte to Alpha - that will matte out that layer based on the alpha of the layer above it.
Alternatively, you could use an effect, I believe it's Set Matte, and then select the matte layer.
Another method that Milenium often mentions but I've never used is to set the matte layer to silohuette alpha - I think the matte layer needs to be set as an adjustment layer.
Are you speaking of rendering? If so, check whether you have enabled to use the Millions+ and RGB+Alpha options in your Output Module settings. For the rest, follow David's advise.
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If you want any layer with transparency to create an alpha channel for all layers then all you have to do is put the layer (like a shape layer or a masked layer) at the very bottom of the composition and then enable Preserve underlying transparency for all layers above. I use this all the time.
You'll find the option just to the right of the blend modes in the Timeline
Or you could put the layer with the mask at the top of the stack and choose Stencil Alpha and leave the T box turned off. I think the first option gives you more flexibility in some projects.
Thanks David, you're steps were very helpful.
The problem I had with this one was my solid had 4 layers below it and 2 above it. So, I built the bottom 4 as a precomp, then built a new comp with the precomp on bottom, solid above it, then the 2 upper layers above that. I then set the "TrkMat" for the precomp to "Alpha Inverted Matte" and bada-bing-bada-boom, problem solved.