Detect missing faces feature explained?

Hi,

When you select some photos, right click, and select "detect missing faces" is this supposed to make iPhoto rescan the photos you've selected and try to find any faces in the photos? As if you re-imported the photos? Or is it just trying to match faces it has already found during import with names on the 'cork board'?

The reason I ask is that I did an experiment of deleting many faces iPhoto found, followed by the "Detect missing faces" bit. None of the faces that were previously found were found again. If I re-import these photos, then the faces are found again.

Cheers,
Chris