Kaio-ken is a powerful technique used by the character Goku in the Dragon Ball series. It allows him to temporarily increase his strength, speed, and agility by drawing on his life force. This technique can be used in various multipliers, such as doubling or tripling his power level, but it comes with risks, including physical strain and potential injury. The Kaio-ken technique is taught to Goku by King Kai, a mentor figure in the series. While it enhances his abilities significantly, it requires careful control and can only be maintained for a limited time, making it a strategic choice in battles.