Impotence Question

Now that I have your attention...my question is about impedance matching in speaker selectors. I have a Bryston 3-way speaker switch box, which does not have impedance matching/balancing. As far as I understand it is a simple three-way switch box with minimal signal path to travel. It has three on/off buttons which operate independently from each other. So I was curious what exactly happens when I switch my 100 watt amp to drive two pairs of 8-ohm monitors simultaneously (yes, besides the fact that both sets of speakers are playing)? Are there sacrifices in performance, and are those sacrifices linear or do they show up in different ways for each pair of speakers? It does not occur to me to drop the volume significantly in one pair of speakers when engaging the second pair which are downstairs, though I imagine this must happen as it's splitting the output of one amp between two pairs of speakers. So what is going on, and how does it differ from a selector which compensates for impedance? I think Niles actually terms their system "Impedance Protection".

Marco