Once the flag is turned on, the directory is re-scanned and the image files are found.
You can use the + or - to scan to see if Sync-O-Matic has found all the slides you think should be there.
It is important not to use this mode on directories which have been created from a PowerPoint-97 Save As HTML process. PowerPoint puts about 8 extraneous GIF images into the directory for various purposes including advertising Internet Explorer. If these directories are used with Liberal Slide Names turned on, these extraneous GIF images will be part of the lecture.