It is really a combination of the long rod and the light line. With a long rod and a heavy line, the line would still hang straight down from the rod tip. With a short rod and light line, you couldn't keep much line off the surface (still OK for very small streams). Rod companies are starting to make long rods for European Nymphing, but they are still mostly just 10 or 11 feet. The most common length for a tenkara rod is probably 12 feet and many are 14 feet.