Kaiō-ken

Kaiō-ken (キングカイ拳, King Kai Fist) is 's down special move in Super Smash Flash 2.

Overview
When a Kaiō-ken is successfully performed, Goku will tint himself with a crimson tone and it will power up his current moveset, making him stronger, faster and more deadly than the 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 about 3% damage. Also if Goku comes in contact with certain objects such as items, the blocks in Mushroom Kingdom III and the wooden breakable platforms in Emerald Cave, they will trigger Kaiō-ken. If there is no elemnts around him, Goku can also hold the button to initiate the attack.

Goku starts taking 1% of damage in just a half of a second (so by 10 seconds after activation he may end up having 20%), and more unfortunately, Goku doesn't 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 exactly five seconds to do so or else the form ends. After that, he must do a chain of 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 very hard to be fully charged, but 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 Viz serialization: "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.

The technique however has its substantial flaws, such as 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, 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.
 * In SSF2 Beta, Goku can also hold the the button to initiate the move and the charge has very less time. However, Goku not can stay form as long the player desires, he still have only 5 seconds of activation and have to attack or suffer attacks to increase the duration.
 * His taunt makes him invincible as he is taking no damage and is not being affected by hazards or items. This was fixed in v0.9.0.2007 update.
 * Even though charging this move does no damage, if its used near a player who is shielding, the shield starts to break.
 * Goku has a pallete which gives him an appearance similar to Kaiō-ken.