ExTuber extract (as well as play and copy) YouTube's FLV files which you have watched from YouTube. Once you've watched video files (.flv) from YouTube, those files are automatically saved in hidden Temp or Cookies directory on your PC as Temporary files for the period. The ExTuber does extract, re-play and copy these files to your hard disk.
There doesn't seem to be a readme file or any real type of "help" tools to help you use it. Just a "Find Hidden FLV files" button on your computer -- clicking that does nothing, even thought I KNOW I have some .flv files here