Kaiō-ken

Kaiō-ken (界王拳, literally: King of the World's fist) is 's down special move in Super Smash Flash 2.

Overview
When a Kaiō-ken charge is successfully performed, Goku will tint himself with a crimson tone and this will power up his current moveset, making him stronger, faster and more deadlier than regular Goku, but not nearly as powerful as Super Saiyan Goku. When initiating Kaiō-ken, Goku lets out a ki aura, any opponent or element that comes in contact with it will have Goku activate it and take 3% damage. The rest of the charges when using it do 5% damage. Also if Goku comes in contact with certain objects such as items, the brick blocks in Mushroom Kingdom III and the wooden breakable platforms in Emerald Cave, they will trigger Kaiō-ken. If there are no elements around him, Goku can also hold the button to initiate the move and the charge has a shorter time.

Goku starts taking 1% of damage in just a fourth of a second (so by 10 seconds after activation he may end up having 40%), and Goku does not gain any kind of knockback resistance. On the upside, he gets a new side special move, Kaiō-ken Attack, and his Kamehameha charges faster. Goku can only power-up when his energy makes contact with an opponent, charging the move or if he flinches by enemy attacks; he has 3.5 seconds to do so or else the form ends. After that, he must chain his attacks to maintain the transformation, because the transformation will disappear if he attacks too late. He also starts taking damage much more quickly. His Kamehameha is hard to charge completely, but almost immediately turns into the third stage of a charged Kamehameha. Also, his smash attacks come out almost instantaneously.

Origin
In the manga and anime, Kaiō-ken is a technique created by North Kaio (北の界王, Kita no Kaiō Funimation adaptation: "King Kai") and is one of two techniques he taught to Goku, the other being Genki Dama, when he trained in the other world. As a result of the Kaiō-ken, Goku's base ki level, as well as his strength and speed, are greatly increased for an instant, however, he later learns to increase the effect of the Kaiō-ken to higher multiplications. When he first mastered the Kaiō-ken, the furthest he could handle was the Kaiō-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 Kaiō-ken attack, such as Kaiō-ken x10. The highest multiplication for the Kaiō-ken ever seen was the Kaiō-ken x20, which Goku used against Frieza. However, after becoming Super Saiyan, Goku discontinued using Kaiō-ken since the Super Saiyan transformation largely overshadows it. It is not until Goku reaches the Super Saiyan Blue form that he is able canalize the Kaiō-Ken while transformed and not endanger his life as a result of the huge amount of energy loss. But still, Goku is not able to hold the transformation for too long and may suffer from severe painful muscle convulsion and an unstable ki disorder afterwards.

The technique however has its substantial flaws. For example, 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 taking the Kaiō-ken x4 to overpower Vegeta's Galick Gun (ギャリック砲, Gyarikku Hou, translated as "Garlic Cannon"), Goku's entire body was thrashed, and he could barely move without hurting himself.

Trivia

 * In v0.7, Kaiō-ken originally require the player to hold down the special move button to initiate this move, then, Goku could stay in the Kaiō-ken form as long as the player desired, in order to return to his normal self, the player would have to press the down special command again.
 * An early glitch made Kaiō-Ken Goku invincible by holding his taunt. This was fixed in the v0.9.0.2007 update.
 * Goku has a palette which gives him an appearance similar to Kaiō-ken.
 * Kaiō-ken Goku's hitboxes are radically different from when he is in his base form; the structure is usually simpler, with only a single large Hurtbox rather than multiple to cover his sprite accurately, and often with a different range and number of active frames in his attacks.
 * In the game itself, the technique is referred to by its English name, "Kaio-Ken", with no macron over the o.
 * Goku can permanently stay in this form in the event match Fist of the World King.