From the anime
Dragonball Z, Kaioken is the fighting technique taught to Goku (the only one to master this technique) by King Kai. Though King Kai created this technique, he admits that not even he is able to fully master it.
The technique is very similar to going super saiyan, although the
power increase is not as
high, it is very effective before Goku first transforms. Goku discontinues using the Kaioken attack after his fight with Pikkon, opting instead to attain
high super saiyan levels.
The technique only last a few seconds according to Goku so you "have to get the job done quickly". It momentarily greatly increases your
speed, strength and abilities, allowing you to achieve a power level unobtainable without the use of this technique.
Through experience and training, the
user can achieve multiple 'levels' of Kaioken, increasing the users abilities even further. However, according to King Kai, if you go up too far, your body will not able to handle it, and all the
mass in your body will be replaced by energy, thereby making it explode. Goku is able to withstand a 4x Kaioken while fighting Vegeta, (putting his fighting
power well over 23,000) in order to counter his Galick
Gun attack. However, after this feat his body is thrashed and has no other way of attacking other than using the spirit bomb, another technique exclusive to Goku and King Kai.
Cell states that he could use this technique if he wanted to, however.
