Kaio-ken 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.
As a result of the Kaio-ken, the user's base power level, strength, speed, and senses greatly increase for an instant. The only change produced by the basic Kaio-ken to be quantified in the series is that it doubles the user's power level. As the series progresses, Goku demonstrates the ability to heighten the effect of the Kaio-ken to increasingly higher multiplications. There is an inconsistency in that Kaio-ken and Kaio-ken x2 appear to have the same effect, despite Goku considering the latter to be the more powerful buffer. Despite this, Kaio-ken x3 and above all consistently multiply the user's power level by the respective multipliers.
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 technique, 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. 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 using 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 to scream out in agony. King Kai tried to master this technique himself but failed and taught Goku instead.
The first time Goku used the Kaio-ken was in the Saiyan Saga when he used it to catch up with and cripple Nappa before he could kill Gohan and Krillin with his Break Cannon mouth blast. Later, while Goku started to battle Vegeta, he realized he couldn't defeat Vegeta even with the Kaio-ken and powered up to Kaio-ken x2 and later x3, raising his battle strength to over 24,000 andoverpowering Vegeta. To prevent Vegeta from destroying the Earth with hisGalick 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 this form 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. Later, in theFrieza Saga, when Goku fought Captain Ginyu, he demonstrated his Kaio-ken in a prolonged way, increasing his battle strength to a power level around 180,000, thus shocking Ginyu and Jeice. After being healed in the rejuvenation chamber following his attack from Vegeta when Goku himself had switched bodies with Ginyu, Goku battled with Frieza using a Kaio-ken once again.
After realizing Goku was stronger than the others he had battled from Ginyu to Piccolo and the rest and even Vegeta, Frieza 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 Frieza, 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 the Kaio-ken x20 and aKamehameha wave combination, which amazingly, Frieza held off with one hand before stopping it with his own blast, barely hurt. A short time later, Goku eventually transformed into a Super Saiyan, which greatly overshadowed the Kaio-ken.Goku's battle against Frieza 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 Saiyancombination (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 forms many times stronger. However, it can be argued that using Kaio-ken in transformed states could seriously weaken 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