Ok, I'll try. a square edge is created when you drill a hole in a piece of metal. The tang is flat then when it comes to the hole, there is a 90 degree angle as measured down through the steel. That 90 degree angle creats what is called a stress riser. By countersinking, or "rounding off the angle" you remove this 90 degree angle and make it more rounded, like the rounded spine on the blade. Did I do ok in explaining it to you, or just confuse you more.