The Kaio-ken ("Fist of the Worlds") was a technique invented by Rokudou Sennin, in which Seireitou , Ryun and Shien were the only people ever able to successfully use the technique. In the series, the Kaio-ken is a technique that multiplies the user's energy for a "heart beat", enabling them to inflict serious damage to foes who are considerably stronger than them.
As a result of the Kaio-ken, the user's base energy level doubles for an instant, however, Seireitou and Ryun later learn to increase the effect of the Kaio-ken to higher multiplications. The topmost multiplication for the Kaio-ken ever seen was the Kaio-ken x1000, which Seireitou used against the insanely powerful Hikaru Kurosaki. The technique also has its substantial flaws, if the user multiplies their energy too much, or if their bodies are too severely damaged, the uprise of energy could easily vaporize them. When Ryun first achieved the form of Kaio-ken, the furthest they could handle was the Kaio-ken X10 and Seireitou could only go up to X15, but through training in the Hall of Heaven, they was able to withstand astounding multiplications of the Kaio-kens, such as Kaio-ken X10 and the Kaio-ken X20, and much later on, Ryun would master up to X500 and Seireitou up to X900 and against Hikaru, he raised it up to X1000, which put a large strain on his body.