This is going to sound like a bit of a noob question (and maybe it is), but hey, I'm happy to learn something new every day, even if it's elementary to others...
 
Say you're listening to a professionally produced song, and there is a small break in the music for all but one instrument (guitar, for instance), and the volume of that instrument increases just for that short break, and then nestles back in amongst the mix when everything else comes back in. A very common occurrence in music, right?
 
Is that transition created during mixing, via manual automation, or is it the result of compression, during mastering?
 
This is something I've wondered about for some time. I've long assumed it to be the former, but could never say with absolute certainty. Would you fine folks be so kind as to enlighten me on what I predict is fairly common knowledge?...
 
Thanks!

That is a mixing decision.  Depending on the context, you may want the solo instrument to get quieter.
 
Mastering is NOT supposed to change what your mix sounds like.

I would also think it would be done with automation, not compression - but as Karyn said, do it in the mixing stage.

I use automated volume envelopes for musical transitions. They bring things in and out of the mix, quite abruptly in some cases, and also adjust the levels during certain parts.....
 
For example, in a chorus, I may want the B3 organ to come up under the mix and be more prominent in the mix....a volume envelope does that just fine.
 
Compression is what I apply generally to the entire mix, but mostly lightly.  No heavy compression for me and I NEVER use compression to control volume levels. Never.
 
My tracks all have volume envelopes in them. The pic below is the piano fills in the first verse of The Best Christmas....   here>>>  http://soundclick.com/share.cfm?id=11971116
 

 
I don't often have an instrument solo while everything else goes silent in my music.... but... I don't think I would necessarily do anything other than mute the others using the envelopes. That solo instrument, MIGHT need to be pulled UP a few db to compensate for the loss of the other instruments, BUT it would also depend greatly on what the instrument was.  What I mean there is that a more delicate instrument like a mandolin might need a few db gain, where drums & bass, for example, would probably be just fine with no changes to the volume envelopes. I seem to recall doing something like that in a song in the past..... but no compression gets used specifically for that, just envelopes. Envelopes are your friends.

If a conscious decision, it is usually automation. I do that all the time, usually just a nudge.  Also move the track to center, which will also often make it sound louder.
 
However, compression can do the same thing - not in average level but in effect.  I was working on a rock song that had a disco ping sound in it.  It was laying under the radar, for the most part, more of a rhythmic element.  Until the end fade out.  It was the last thing left and even gentle buss compression popped it up, way too loud.  I ended up automating it lower.  Now such is just one more thing to be aware of.
 
@

Thanks a bunch, folks

This reminds me of something Charles Dye mentions on Mix it Like a Record, which he calls the "too loud rule", using automation to create that short peak somewhere in a song. Helps breathe life into a mix.