Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
When desktop effects are enabled, extenders "roll up" and "roll back out" instead of just suddenly appearing and disappearing. This goes along with the idea of "organic interfaces" where things shouldn't exhibit magical behaviors: if you saw someone disappear into thing air in front of you .. well .. they'd either be an illusionist playing games with you or else Bilbo Baggins mucking about with his magic ring. It certainly wouldn't feel "normal" though, because that's not how thing behave according to our accumulated sensory experiences.
It doesn't work with QWidget based popups yet though there's no reason it couldn't; Plasma::Dialog just doesn't have the logic for it yet. That means that things like kickoff or the device notifie still behave a bit magically.
I'm pretty sure that when we do extend it to all "things that come out of an edge docked panel" that we'll need to make it configurable (though it turns off automatically when animations are cut in plasma as a whole or when there is no compositer around) since some people will likely absolutely hate it for reasons of personal taste. I think it's jolly good (to use a quaint Britishism), but I can just tell it's the sort of thing someone, somewhere will take umbrage with.
It also occurs to me that this would be so much better as a compositing plugin.