I also have done this on sites, after initally being sceptical, then having it demonstarted and having a go myself with success, unsuprsingly upon retrying under controlled conditions with no visual clues it did not work. This does not lessen its usefulness but I suggest it does not nothing but tap into a number of visual and other clues that you may not consider directly. Be interested to hear if anyone has researched this. That is my hyopthesis anyway, if anyone has anything beyond 'it is magic and therefore cannot be tested' i would be interested.