A bo-hi (also spelled 'bohi,' 'hi,' 'fuller,' or 'blood groove') is a groove in the blade placed directly behind the spine that helps to reduce weight without sacrificing durability. They are commonly found on folding knives made before the advent of stainless steel, when aluminum was used instead. Katana had these grooves also. However, they are not present on all katanas. You can see from this image that the back side of the blade is completely smooth: Image courtesy of http://www.foldingknife.com/ Even though it's not visible in this photo, most katana blades were actually double-edged, meaning they had a sharp edge on both sides of the blade. This allows you to use either edge for slashing and stabbing actions. A single-edge knife is more suitable for detailed work like filleting because you want only one cutting surface to keep track of. There are some exceptions such as the yokoto, which was usually only equipped with one side of the blade; but even so, it was still considered offensive weaponry. So yes, Katana had bo-hi on their blades too. But not all ones did. And since this is true, it means that Mr. Miyagi didn't invent anything new when he made his knife.