Kaio-ken (界王拳, "Fist of the World King" or "World King Fist") is a fighting technique invented by King Kai. However, Goku is the only person ever able to successfully use it. It is a technique that multiplies the user's ki for a "heart beat", enabling them to inflict serious damage to opponents 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. While in the state, the ki aura's color becomes crimson.