Kaiō-ken

Kaiō-ken is 's down special move in Super Smash Flash 2.

Overview
When performed, Goku lets out a blue ki aura around himself, and any opponent or other hurtbox that comes in contact with it will have Goku activate the move and take 3% damage. Once activated, Goku temporarily transforms into his Kaiō-ken form, in which he is tinted with a crimson tone and his moveset is changed.

While in his Kaiō-ken form, most of Goku's moves are stronger and faster versions of his moves from his base form. For example, his Kamehameha becomes Kaiō-ken Kamehameha, which charges twice as fast and is more powerful overall. His smash attacks are also more powerful and cannot be charged. Many of his moves also have different properties, and others are changed entirely. The most notable of these is his side special move, which changes from Ki Blasts to Kaiō-ken Attack, in which Goku dashes forward while surrounded by energy. Despite these strengths, Goku takes 1% recoil damage every fourth of a second in this form, and he does not gain any kind of knockback resistance. Additionally, Goku must continuously land attacks on opponents to maintain this form, as it will end if he does not attack any opponents after a certain period of time.

Kaiō-ken Goku's own version of the down special move releases red ki aura around him that deals 5% damage to opponents hit, but it cannot be held.

Origin
Kaiō-ken is a technique in the Dragon Ball franchise created by North Kaio 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, and 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, as 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. However, 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.

Despite Kaiō-ken's power, 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 Garlic Cannon, Goku's entire body was thrashed, and he could barely move without hurting himself.

The attack's ability to damage opponents who touch Goku may be a reference to various Dragon Ball games, where the opponent would be blown back or take damage if a player character transforms into another form while close-by. The Dragon Ball Z: Budokai series mirrors how the attack works in SSF2 most notably, as Goku also had a Kaiō-ken transformation. However, no game in the Dragon Ball series required the player to "hit" the opponent to transform.

Trivia

 * In v0.7 of the demo, Kaiō-ken originally required the player to keep using the move for about four seconds straight to activate, and Goku could then stay in the Kaiō-ken form for as long as the player desired. In order to return to his normal form, the player would have to use the down special move again.
 * Goku's red palette swap gives him an appearance similar to his Kaiō-ken form.
 * 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 his Kaiō-ken form in event #37, Fist of the World King.