I saw a documentary on this a few years ago. The visual cues can be very subtle, and they do the sorting at insane speeds. The chick is litteraly flighing to one bin or the other, and just stops a fragment of a second in front of the guy. This, with an error rate ridiculously low (like one in a few thousands or something like that).

I believe the operator really doesn't know how he parses the information, and at these speed and error rates it must be something more than seeing bumps or not.