Kaioken

Kaioken is a fighting technique invented by King Kai. However, Goku is the only person ever able to successfully use it. Then in the series, he teaches it to the younger Saiyans. 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 Dragon Ball Z animated series and Manga. The user's aura becomes a reddish color.

Usage in Canon
The Saiyan Fighters have used this technique many times during the series. At the beginning of the series, the Kaioken ability was only used up to Kaioken x2-4. But in episode 6, "Shadoo of the Past", the Evil Trio were beginning to overpower the heroes. As a result, they were forced to push it up to Kaio-ken x6.

But as time went on, the fighters developed an endurance to the Kaioken (Seeing as it takes much energy and strength to subtain it), allowing them to go up to Kaioken x10. This transformation is used throughout the entire duration of the Mecha Saga and for most of the Redux Saga. The transformation is mainly used at x10, x12 or x20 for the battles in these arcs. However, in the fight against the PowerPunk Girls in Episode 38, it has been implied that they have discontinued use of the Kaio-ken, seeing how vastly superior the False Super Saiyan is to the transformation.

Power
The Kaioken levels have been stated to go from 1-20. Though the effects go x1.5-x20. 20 being the highest achived. If the fighter can withstand the energy level of Kaioken, no strain is felt, unless they overuse the Kaioken. An example being in Episode 12, when the RowdyRuff Boys use the Kaioken x20 to thwart Bowser's Dark from, but are thrashed by their incomplete use of the form.