You have to offset the "eye"/"camera" position that the sky is rendered from based on the vector between the "main" (player) camera and the plane it is projected onto
This allows you to "peek" to see things that lie behind its edges if you just look at it straight-on. For moving the sky you can just rotate the eye/camera rendering it.

ah, that makes sense.

you'd have to check the article - that's the word the person used. i don't know if it's actually the correct term.

What other plane...?

It's really just a render of the skysphere for the target level pasted onto a quad at the back (or center if they are accessible from both sides) of those portals. I didn't read the blog entry, it seemed pretty lengthy, just thought I'd summarize it for other lazy people. Still, it is a nice effect indeed

but wait, for some of the portals (especially in Spyro 1), the sky moves within the portal. how would that work?

(in general, i'm finding myself confused over how a separate plane can exist inside of another one (as seen in the article), but that's okay.)