There was a film many years ago with Bear sharpening a head. What the poster may be referring to was the final strokes Bear used which more or less using the file like a steel to correct or remove the burr. I think what one needs to watch out for with softer metal heads like the old Bears is a smooth bur, it may cut the hair on your arm, but that bur edge may not hold up going cross ways through an animals hair and hide or rib bones. Stropping the head with something like a jewel stick or a belt or even the file will reveal what edge would be left when that bur comes off. A bur and microscopic cutting teeth are not the same thing, even a razor blade has those cutting teeth. Like my son demonstrated one time when showing somehow how he sharpened his Grizzlies, turning the head in the sun light to show the bur, he said see that, it is almost a magnetic thing. Then he took a hardware magnet that farmers use for cows, ran it lightly across the head, and held up the magnet, "There's your bur."