When you open a torrent and give BitTorrent a filename/directory that already exists, it must check the file to see how much of it is useful data and how much is junk. (Recall that BT allocates space for the entire file when you first start a torrent.) To do this it must read the entire contents of the file, and generate what's known as a hash for each piece of the file. A hash is a cryptographic function that creates a small summary or digest of a large amount of data. BitTorrent uses the SHA hash function to determine which parts of the file are good and which are bad.