I can't even cut a straight line in balsa wood. Think of the practice it must have taken to saw a log into planks with hand tools, especially when one's work surface was the narrow inclined log itself.

But this photo gives us a clearer idea of how the Japanese processed logs in the era before mechanized sawmills. If you look closely at the end of the log, you can see that parallel lines have been scribed on the surface. You can also see black dots that must have played a role in marking the cut lines.