The AGC (Automatic Gain Control) basically drops the volume when it reaches the Threshold level you set. After that, it boosts the gain, attempting to keep everything level. Some hardware compressors call it "Gain Make-Up", meaning it boosts it back up to compensate for the compression.
In VDJ, I use a two-stage compression, where you of course can tweak the settings of both stages. The first stage is intended to be a slower-acting compression, just to keep the overall volume relatively even. The second stage catches the peaks (like a hard drum hit after a quiet part), and working in tandem, it generally keeps the volume even with no surprise loudness.
The "Enhance" adds back some high treble during compression. The point of this is to prevent a phenomenon called "pumping", where everything drops out every time the kick drum or bass line hits. This feature was introduced in the 1970's when Disco became popular, but it helps make the compression more transparent for most music in general.
The idea is this: You're at a gig, people are talking, other noises are happening. Your music being at a sane level and being well controlled helps to overcome this noise, and they can hear the quiet parts of a song as well since it will be brought up.
When I said it's like an FM radio station, what I actually meant was that they use heavy compression mostly to compensate for road noise, since most of their listeners are in their car. So much like road noise, the AGC feature helps overcome conversation noise at a gig. Has not much to do with the solar issues as much as just being louder (perceptually) than the next station.
Most newer music is already overly compressed as it is... but the gentle compression (with the default settings) in VDJ just helps overcome crowd noise.