I have been using uTorrent for a long time. I have used its "force recheck" feature countless time. Every time it surprises me how uTorrent successfully detects which parts of the file had been downloaded and which parts are yet to be downloaded. It checks them even if you are not connected to internet; so I presume that it doesn't download any data from internet to do this check.
Can you please explain me how this "force recheck" feature works. I think that a hash for every single piece is kept in the .torrent file; but I'm not sure, it is just a guess. I searched in Google with keywords "torrent file format" to find the file format of .torrent files, but instead it returned a lot of torrent search results. So Google is not my friend this time. Please explain it to me.