From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Kaio-ken (界王拳, Kaiō-ken; Literally meaning "World King Fist") is a supportive technique invented by North Kaio. Son 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 manga and anime. While in this state, the ki aura's color becomes crimson.
As a result of the Kaio-ken, the user's base ki level greatly increases for an instant, however, Goku later learns to increase the effect of the Kaio-ken to higher multiplications. When Goku first mastered the Kaio-ken, the furthest he could handle was the Kaio-ken x4, but through training at 100x gravity and then later with his dramatic strength increase after his fight with Captain Ginyu, he was able to withstand astounding multiplications of the Kaio-ken, such as Kaio-ken x10. The highest multiplication for the Kaio-ken ever seen was the Kaio-ken x20, which Goku used against Frieza.
The technique however has its substantial flaws, such as if the user multiplies their base ki too much, the uprise of ki could easily obliterate them, or severely damage their body, as it did Goku's; after taking the Kaio-ken x4 to overpower Vegeta's Galick Gun, Goku's entire body was thrashed, and he could barely move without hurting himself. Even one friendly slap on the back from a gracious Yajirobe caused Goku severe pain.
Dragon Ball Z
The first time Goku used the Kaio-ken was when he used it in the Saiyan Saga to catch up with and cripple Nappa before he could kill Gohan and Krillin with his mouth blast. Later, after Goku realized he could not defeat Vegeta even with the Kaio-ken x2, he powered up to Kaio-ken x3, raising his battle strength to near 24,000 and overpowering Vegeta. To prevent Vegeta from destroying Earth with his Galick Gun, he used a Kaio-ken x3 Kamehameha, a combination of the Kaio-ken x3 and the Kamehameha, in an attempt to save himself and the Earth, initiating an energy wave struggle. When both of their attacks proved unable to overtake the other, Goku used the Kaio-ken x4, surging through Vegeta's Galick Gun and blasting him into the sky. Goku also used the technique briefly when he fought Vegeta's Great Ape form, but was whipped aside by Vegeta's tail, surprised at Vegeta for remaining as agile as ever even in the huge size of the Great Ape transformation. After blinding the Great Ape Vegeta with the Solar Flare technique, Goku uses the Kaio-ken one last time during the battle to help distance himself fast from the stunned Vegeta and buy time to form a Spirit Bomb.
Later, during the Frieza Saga, when Goku fought against [Commander [Ginyu]], he demonstrated his Kaio-ken in a prolonged way, increasing his battle strength to a power level around 180,000, thus shocking Ginyu. After being healed in the rejuvenation chamber following his attack from Vegeta when Goku himself had switched bodies with Ginyu, Goku battled with Freeza with almost equal power, thanks to his Saiyan nature's ability to grow stronger after recovering from serious injury. After realizing Goku was stronger than the others he had battled from Ginyu to Piccolo and even Vegeta, Freeza used 50% of his maximum power, which gave him an enormous power boost. Goku then had to resort to the Kaio-ken x10, to even have a chance at battling Freeza, but even this wasn't nearly enough. Goku then attempted to use the Kaio-ken x20, which gave him near equal power to the tyrant. Goku eventually used a Kaio-ken x20 Kamehameha combination, which amazingly Freeza held off with one hand before stopping it with his own blast, barely hurt. A short time later, Goku transformed into a Super Saiyan, which greatly overshadowed the Kaio-ken. However, since – according to battle powers given in the Dragon Ball: Daizenshū companion released in Japan – becoming a Super Saiyan increases one's strength fifty times, it is unknown if a Kaio-ken x49 would have equaled the power of a Super Saiyan (if such a Kaio-ken level is even possible to pull off).
Goku's battle against Freeza is the last time the Kaio-ken is used in the manga, as the introduction of Super Saiyan outclassed it. However, in the anime filler fight against Pikkon, Goku used a Kaio-ken and Full Power Super Saiyan combination (something he calls a Super Kaio-ken), which gave him an incredible power increase. Because of this one instance, it appears that Goku can also demonstrate the Kaio-ken's usability in transformed states, which potentially means he can make his transformed states many times stronger. However, it can be argued that using Kaio-ken in transformed states could seriously injure or even kill him. This is evident in that the already-dead Goku was exhausted after performing the technique for only a few seconds during his fight with Pikkon.
The World's Strongest
- The Kaio-ken appeared in Dragon Ball Z: The World's Strongest. He first uses it to deflate the bio-warrior Misokatsun. He used it again soon after to free himself after being frozen solid by Ebifurya, and then easily took out both Ebifurya and Kishime with it. He later used the Kaio-ken x3 to attack Doctor Uiro's giant robotic body and succeeded in breaking off Wheelo's right arm. The two then engaged in an Energy Struggle similar to the one Goku and Vegeta had during the Saiyan Saga. As in that struggle, Goku overcame Wheelo's blast by using the Kaio-ken x4.
The Tree of Might
- The Kaio-ken appeared in Dragon Ball Z: The Tree of Might. Goku first used the Kaio-ken to avoid getting hit by Great Ape Gohan, but was eventually knocked down by him. He later used it to take out the Crusher Tullece Corps. After Tullece became more powerful when he ate the Fruit of the Shinseiju, Goku used the Kaio-ken x10 (x20 in the FUNimation dub) to attack him, but was still unable to keep up.
- The Kaio-ken appeared in Dragon Ball Z: Lord Slug. Goku used the Kaio-ken to rip a hole through Giant Slugg after Piccolo had given him some of his power.
Dragon Ball Z: Cooler's Revenge
- The Kaio-ken appeared in Dragon Ball Z: Cooler's Revenge. After Cooler transformed and became far more powerful, Goku used the Kaio-ken to attack him with no success. Cooler even flew through Goku's Kaio-ken Kamehameha combo.
The Return of Cooler
- The Kaio-ken appeared in Dragon Ball Z: The Return of Cooler. Goku used the Kaio-ken against Meta Cooler, but it proved far too feeble. Goku had to transform into a Super Saiyan soon after.
Goku has used the Kaio-ken in many Dragon Ball video games, usually as either a temporary power-up or physical attack like in the manga/anime, or even as a prolonged transformed state similar to his Super Saiyan forms.
In Krillin's "what if" story in Dragon Ball Z: Supersonic Warriors, after dying of a Heart Virus, Goku (with Baba's help) returns to teach the Kaio-ken and the Spirit Bomb techniques to Krillin so he can protect Earth in Goku's place. Krillin can use these techniques in his level 3 form.
Dragon Ball Online
In Dragon Ball Online, set in 1000 Age, it is said that between the end of the original manga and the start of the game North Kaio taught the Kaio-ken to the inhabitants of Earth. This included the Humans, the planet's original inhabitants who since the manga's end have began practicing different forms of martial arts, and the Namekians, who fled to Earth after their planet was destroyed by Miira. The Majin race, who were not taught the technique, managed to learn it through mimicry, similar to how Majin Buu learned Goku's Kamehameha by witnessing it.
The appearance and effects of the Kaio-ken in the game are very similar to how it appears in the anime; activating it engulfs the player in a flaring red aura. The self buff increases the players' movement speed, allowing them to run faster, and in combat: their attack speed, chance of critical strike, and maximum attack power. However, the only increase that comes with multiplying the Kaio-ken buff is the chance of critical strike. (i.e.; activating it doubles (x2) the player's critical strike, and the following multiplication would put the increase at x3). During its use, both the players LP and EP are drained and cannot be deactivated manually, but instead deactivates itself when HP is considerably low. Kaio-ken also has a six-minute cooldown period.
Furthermore, the Kaio-ken technique can be amplified over x20, as never seen in previous uses by Goku in the original series, and due to it being accessible to all classes thus far, it can be used to increase the strength of trademark techniques other than the Kamehameha. (for example, Wolf Fang Fist, Destructo Disk, Hellzone Grenade, Big Bang Attack, and Special Beam Cannon).
- During the Garlic Jr. Saga, Goku's wife, Son Chichi, imitates the technique's signature red-colored aura after being angered by Kuririn's girlfriend, Maron. Despite the aura's appearance, it is not the Kaio-ken, but mimics it to represent Chichi's immense rage.
- In Dragon Ball Z: Budokai and Dragon Ball Z: Budokai 2, the Kaio-ken is dubbed the "King Kai Fist", a rough translation at best.
- The attack has been usually pronounced incorrectly in the English versions of both the anime and video games ("KAY-oh-ken" rather than "KAI-oh-ken"). However, Peter Kelamis (Goku's second voice actor in the Westwood dub) pronounces it properly in the movies The World's Strongest and The Tree of Might, and Sean Schemmel (Goku's adult voice actor in the FUNimation dub) pronounces it properly in Dragon Ball Z: Burst Limit, Dragon Ball Z: Tenkaichi Tag Team, Dragon Ball: Raging Blast 2, and Dragon Ball Z: Ultimate Tenkaichi, as well as for the Kaio-ken multipliers in Dragon Ball: Raging Blast. The technique is also pronounced correctly in the FUNimation Kai dub of Dragon Ball Z Kai.