Kirkd, it's not like an expander, for two reasons: First, it is reducing the overall dymamic range, like any compressor instead of increasing it like an expander; second (and what makes it different from normal compression or expansion) it only does it's thing on the bottom of the dynamic range it is effecting, and leaves the top of the dynamics untouched.
I started exploring parallel compression because I often felt that I wanted a different EQ setting for the quiet portions and the loud parts, and didn't want to have to keep turning up the volume to hear the details and back down for the loud parts.
The agressively compressed copy of the stereo track EQ'ed with a loudness-countour-like curve that dramatically cuts the mids and boosts the highs and lows might not sound good on it's own, but is mixed in just enough to reinforce the quiet parts. Because the compressed copy has less dynamic range than the original, it's contribution is progressively less as the music becomes louder, blending in smoothly. Plus only a small portion of the end signal is actually passed through the compressor, instead of all of it.