I don't know the specifics but I imagine most of it is waving a magic wand and saying poof now this room is classified. But there are logistics that go with that, certain door technologies that have to be in place, probably some complex security procedure for "switching" between modes, (i.e. I would think they need to clear the building of uncleared personnel and be 100% sure there isn't someone hiding in a bathroom somewhere) etc etc, and it's enough of a pain that most buildings are either one or the other all the time. The ability to switch on the fly for a large facility like that is super rare and indicative of there being a real need for switching.

All I know for sure is on the tour they mention they can switch the whole building to be unclassified or classified and during the tour it is in unclassified mode.