Goku has gained a lot of powerups since the start of Dragon Ball Z. He now has access to many forms and techniques for making himself stronger, including Super Saiyans 1-3, Super Saiyan God, Super Saiyan Blue and Ultra Instinct. Going back to the start of DBZ, before the concept of Super Saiyans was even brought up, Goku had another powerup technique that he relied on -- the Kaio-ken.
The Kaioh-ken used to be Goku's go-to powerup. It can multiply his strength and speed in short bursts. It became largely obsolete with the introduction of Super Saiyans, which is why it wasn't really used again until Dragon Ball Super. However, the technique itself is still pretty cool and deserves more attention. In honor of the awesomeness of the Kaio-ken, here's a look at what it does, where Goku learned it and how he uses it.
It all started during Goku's time preparing for the arrival of Vegeta and Nappa. In order to obtain the strength to fight them, he ventured to King Kai's planet in the hopes of being trained by him. After putting Goku through some trials, the Lord of Worlds concluded that the Saiyan had what it took to master both the Kaio-ken and the Spirit Bomb, so the two of them began training right away.
Unfortunately, very little of Goku's time spent learning the Kaio-ken is actually shown on-screen. The training is only seen briefly during the recap for Episode 21, "Come Forth, Shenron! The Saiyans Finally Arrive on Earth." During the recap, Goku is briefly shown practicing the technique alongside King Kai on some rocks. King Kai gets worn out and has to stop, but Goku is able to keep going and complete the training session, thus displaying his amazing potential.
The Kaio-ken serves to strengthen the user in the instant that it's used. It can be used for extended periods and can even multiply one's power by as much as one wants, but it's risky. If it's used too much, it can destroy one's body and end their fighting career.
Goku first successfully used the technique against Nappa. Before the malevolent Saiyan can attack Krillin and Gohan, Goku used the technique to catch up to Nappa and break his back. This isn't bad for a first try using it in actual combat.
Goku's second time using the Kaio-ken didn't go as smoothly. Against Vegeta, Goku was forced to use the Kaio-ken x3 and, eventually x4, just to force Vegeta back. This was bad, as not only did Goku have to use the technique for an extended period of time, but his body wasn't strong enough to handle more than a Kaio-ken x2. Even if Vegeta hadn't later crushed his body, he still would have been left in agonizing pain.
Goku got another good opportunity to use his Kaio-ken against Frieza. In this fight, Goku, bursting with energy and newfound resolve, launched a Kamehameha in tandem with a Kaio-ken x20 to blast his enemy for the sake of his friends. It was the first time Goku did any real damage to Frieza in his final form, which also wounded his ego.
Goku was eventually able to overwhelm Frieza with his new Super Saiyan transformation. Not only did this form make Goku stronger than he ever could have been with the Kaio-ken, but it apparently didn't come with any of the drawbacks of the technique. With the introduction of this superior powerup, the Kaio-ken was shelved for the rest of DBZ.
The Kaio-ken wouldn't be used canonically again until the Universe 6 saga of DBS. According to Goku, he had to stop using the technique because it wouldn't work alongside Super Saiyan. Since Super Saiyan Blue works fundamentally differently from Super Saiyan, the technique became accessible again. Goku's intense new blue and red state gave him the power necessary to take on powerful opponents like Hit and Zamasu.
As impractical as it was for a while, Kaio-ken is certainly one of Goku's cooler techniques. It gives him an awesome red glow, has its own sound effects and almost always has a spectacular showing. King Kai's penultimate technique deserves all the respect it gets, both in-universe and out.
