How are the intensional refsets maintained?

I've just watched the demo of the intensional refset functionality (http://screencast.com/t/SRO8Fu5G5 )
And wondered how these refsets 'recalculate' as content is added?
Specifically, i noticed than when inclusions and exclusions are made, the definition doesn't appear to change.
(At least from what I can see in the demo?)