Let's say you have an Artist pane. And in that Artist pane you have <unassigned> and Metallica. (for simplicity).
If you select <unassigned> you will get a list of files with no Artist set for them. If you then click the Quick Tag button you'll see the little tag icon appear next to <unassigned> and Metallica in the Artist pane. There's also another line that'll say <new artist>. You'll notice that the icon next to <unassigned> is filled in while the icon next to Metallica is empty. If you then click the tag icon next to Metallica, all of the files you have selected will be given Metallica as their artist.
It works really nice except for a couple of minor inconvienences.
1. There's no way to then browse to a different set of files without toggling Quick Tag mode. Sure, it's only one extra click, but for some reason that one extra click seems to introduce enormous amounts of slowdown during heavy tagging sessions.
2. If, because of the set of files your looking at, the value you want to set for a field is not displayed, even though you know you've used it before elsewhere, you still have to select <new artist> in order to set that tag.