Kaio-ken (界王拳, Kaiōken) is a technique created by King Kai and used only by Goku. It is a power-up technique that multiplies the user's ki, strength, speed, and reflexes, but can cause serious damage to the user's body, making it more like a double-edge sword. While in the state, the user's aura color becomes crimson.
Running gag
In DragonBall Z Abridged, there is a running gag in whenever Goku shouts "Kaio-ken!", someone else will ask "Kaio-What?" and receive a blow usually to the face by Goku or someone else. So far, this happens to Vegeta (three times), Captain Ginyu while inside Goku's body, Lord Slug (he says the "Kaio" line while Piccolo says "What" line), Bardock (he gets a vision of his son using the technique and gets caught off-guard), Freeza, and Dr. Wheelo. This gag was somewhat averted with Freeza as Goku fails to scathe him.