[NoEgo]

One thing I can say is this. If you play a held organ note through a comp it changes the way it sounds. The harmonic structure is what creates a note of any complex waveform such as voice, whatever. Those harmonics sound at different levels around the fundamental ....therefore if the note is compressed rather than just maintained in volume as a unit...the note will take on a new harmonic structure. Voila ...why a compressor is different.

make sense?

Yeah the Waves answer was kind a lame.....but you do need to have the music side chained so the level of the vocal or whatever track you are using it on can read the other tracks..this is interesting because they advertise it as taking place of the real engineer riding faders....well...the latter...that is done in real time tracking as much as mixing.
The Vocal Rider seems to be for the mix....more so.....and to sit the vocal in the mix only I suspect. Not to get the best single track level.