Kaiō-ken

Kaiō-ken (界王拳, "World King Fist") is Goku's down special move in Super Smash Flash 2. To achieve the Kaiō-ken, Goku must charge his power for about 5 seconds without moving, any opponent that touches Goku while powering up, will receive minor knockback, but no damage. 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, but not nearly as powerful as Super Saiyan Goku.

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, Goku later learns to increase the effect of the Kaiō-ken to higher multiplications. When Goku 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.

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 Cannon, Goku's entire body was thrashed, and he could barely move without hurting himself.

In Super Smash Flash 2, just like in the Anime and Manga, this transformation, has its costs, as Goku starts taking 5% of damage in just a half of a second (so by 10 seconds of playing as him you'll have 100%), more unfortunately, Goku doesn't gain any kind of knockback resistance. He gains a new side special move, Kaiō-ken Attack, and a second teleportation on top of his Instant Transmission Kick, and only his Kamehameha remains unchanged. To return to his former self, simply the down special move should be used again to remove Kaiō-ken completely.

Trivia

 * To make Kaiō-ken look redder, do a Team Battle with Goku on the red team. Once you are in the battle, go Kaiō-ken and Goku appears redder. For a yellowish Kaiō-ken, repeat the redder process, but have Goku on the green team, and being on blue team makes Kaiō-ken look purple.
 * His taunt makes him invincible as he is taking no damage and is not being affected by hazards or items.