The Reincarnation technique is a Kinjutsu technique developed by Joshin that can reincarnate the users body. After being with Orochimaru for a few years, Joshin was able to obtain many things, and while there Joshin experimented with things himself. Including this technique; in Orochimaru's hideouts were many things, techniques, secrets of the soul and body. Joshin had access to just about all of this stuff, so thats what he did. Joshin read up on a lot of things and after doing that he developed this technique. In situations where Joshin cannot do anything, he uses this technique.