It's just a (slightly) different way of looking at things. The "mixing" in CEP, in my opinion, is what you do when you're playing all the tracks back and getting the levels and panning the way you want them. The "mixdown" button is just the last thing you push to make that happen once you've already "mixed" in preparation for the processing.
I've done my fair share of "mixing" on a 4 track, where I've had one track that might have an electric guitar intro at the beginning of a track, then acoustic rhythm track for a couple of minutes, then an electric lead, then more acoustic, then maybe a bg vocal at the end...(sound familiar?). Anyways, mixing was a very ACTIVE process as I had to jack with the level meters in real time knowing that the electric guitar solo was WAY louder than the acoustic rhythm part, so I had to remember to jump on that fader at 1:56 or I'd have to start over.
In CEP, the envelopes can act much like automated faders (which, like automated faders) you set up to trigger at certain points in the mixdown.
There is a virtual mixer in CEP 2.0 (actually, there's one in 1.2a if you want one...just open up the individual sliders for each of the tracks and line them up next to each other on the screen..PRESTO, "virtual mixer"). Anyway, in 2.0, under "View" in Muti-track, just click on "Show Mixers Window." In fact, this is the only way I've been able to see the master slider that used to just be there in CEP 1.2a. This feature lines all the track faders up next to each other so you can get rough levels. Then, once I've got them close, I drag that mixer window all the way over to the left so that it just shows the master slider, and I use the "envelopes" discussed earlier to bump or reduce a track's volume just in certain places (ie., the automated faders).
That's digital "mixing." The "mixdown" button is just the processing of what you've already mixed. And I definitely mix in real time...I'm just not recording it permanently until it's ready for the "mixdown" button