The Kaio-ken ("Fist of the Worlds") or the Kaio-Ken Attack was a technique invented by King Kai, in which Goku was the only person ever able to successfully use the technique. In the series, the Kaio-ken is a technique that multiplies the user's ki for a "heart beat", enabling them to inflict serious damage to foes who are considerably stronger than them. After the introduction of the Kaio-ken, it became one of Goku's signature attacks, however, it became greatly overshadowed by his ability to become a Super Saiyan later on in the animated series and manga.