One of Goku's most well-known abilities in the world of Dragon Ball is his Kaio-ken technique, but what exactly is it, and how does it work? Though it was one of the first skills learned by Goku in Dragon Ball Z, it somehow continues to be one of his most effective weapons in combat.
After Goku was killed in the battle with Raditz in the Saiyan Saga, Goku traveled to King Kai's planet where he went through a vigorous training process that involved numerous tests and exercises. Finally, King Kai decided to teach Goku the Kaio-ken technique, an ability that heightens Goku's abilities and gives him a red aura. After being resurrected by the Dragon Balls, Goku returned to Earth to help his friends defeat Nappa and Vegeta. Goku's use of Kaio-ken against Nappa demonstrated how powerful he had become. The ability also played a key role in his ensuing battle with Vegeta. It was used again in the Frieza Saga, but ignored in later battles. Years later, it was revived in Dragon Ball Super.
The Kaio-ken technique is one of Goku's most popular moves, but there are some questions over how it works and what makes it so special. After all, many abilities lose their effectiveness as the series go on and characters get stronger, but Goku has yet to abandon Kaio-ken. That's because Kaio-ken is a technique that acts as a multiplier of sorts, which means it can make Goku stronger regardless of his current power level.
When Goku activates Kaio-ken, it doubles his power, strength, speed, and senses, all at the same time. Since doubling someone's abilities understandably takes a considerable toll on one's body, it makes sense that Goku is only supposed to use it for short lengths of time. That's why it's only active for a brief moment when he attacks Nappa. After multiple uses, Goku becomes accustomed to it, and uses it longer - even though the effects of this can be detrimental to his body. Also, Goku is capable of pushing the limits of Kaio-ken and multiplying his power even further. He can use the Kaio-ken to multiply his power by 3, 4, 5, 10, 20, or even 100. However, going too far can kill Goku, since his body can only hold so much energy.
So why does Goku take a break from using Kaio-ken after the Freiza Saga? This is explained by the fact that Goku's primary ability in the Cell Saga and Buu Saga is his Super Saiyan transformation, which isn't compatible with Kaio-ken. According to Old Kai, both forms cause intense strain on the body. Super Saiyan Blue, however, works differently, and can be used in unison with Kaio-ken. So unlike Goku's Super Saiyan 3 transformation, the Kaio-ken is able to remain relevant in the anime.