If they know what they're doing, and why they're doing it, they're feeling for the "feather edge", or more precisely, for the lack of it. To get a really fine edge, you pull out a VERY fine burr on the edge of the knife, fold it over, then take it off, leaving a "sharper than a razor" edge. I don't know how to explain it any better than that, but that's what Pop taught me thirty-some years ago, and it's a lot easier to grasp with a demonstration than with written words (and takes quite a bit of practice to get good at).
Troy