Genryu is the Japanese word for headwaters. Picture narrow canyons, high gradient streams, no roads and often, no room to cast. The fishing style is often lantern fishing, which is a very short line technique. The style is called lantern fishing because the long rod and very short line are reminiscent of the way lanterns were carried hundreds of years ago (the lantern was tied to a short line, which in turn was tied to the end of a long bamboo pole).
Rather than using a very short rod for casting in the tight quarters, which we generally do in the US, the Japanese use a long rod and short line, essentially lowering their bait or fly directly into eddies or current seams or using underhanded "pendulum" casts to get a bit more distance.