Find Sources Now, looks for identical copies of the file being downloaded. It compares File Hashes (Mathematical algorithms) not filenames for a match, So files that have been completely misnamed can still be found and downloaded from. It is far more accurate than simply searching for Similar.
This essentially does the same thing as the Search for Alternates option. The primary difference it that it does so directly from the transfer window rather than searching in a search window of its own. It also automatically adds them to the queue if they meet your AFS/AEQ criteria.