Kaiô-ken

Kaiô-ken is technique from the manga Dragon Ball/Dragon Ball Z and the anime Dragon Ball Z. It allows a character to multiply their speed and power, often letting them work on an equal level against an initially more powerful foe. The Kaiô-ken is dangerous though, and does damage (potentially fatal) to the user's body while they're in that state.

Son Goku is most noted for using this technique (to great extremes at times), but pretty much stops using it once he is able to access the more potent Super Saiyan state.

In Dragon Ball Z: Budokai, this technique is called the King Kai Fist