Ever since Goku went Super Saiyan back during the Frieza saga, he hasn't had much use for his old Kaio-ken technique. That changes in the most recent episode of Dragon Ball Super.
In a battle against Universe 6's Hit, Goku is forced to resort to a powerful new form to overcome his opponent. That form is none other than Super Saiyan Blue Kaio-Ken. It sounds impossible, but the show actually goes out of its way to explain how it works.
Every Dragon Ball Z fan knows Goku stopped using Kaoi-ken upon achieving Super Saiyan status for a number of reasons. The first, and most obvious, is that Goku's Super Saiyan form simply outclassed the power of Kaoi-ken. Though the ability allowed Goku to boost his base power all the way up to a multiplier of 10, there was simply no reason to use it by itself when he could simply go Super Saiyan instead. The other reason, however, is that Kaoi-ken couldn't be used in conjunction with Super Saiyan, either. The technique would put too much strain on the user's body, and without near-perfect ki control, would instantly kill the user.
Not that it stops Goku from trying. Prior to his battle with Hit, Goku had been attempting to perfect the technique to use against god of destruction Beerus. Using his new Super Saiyan God powers, Goku estimates during his battle with Hit that he has a roughly 10 percent chance of surviving the use of Kaio-ken while in Super Saiyan Blue form.
Of course, it works. Goku is able to boost his Super Saiyan Blue power by using Kaio-ken x10, allowing Goku to move in stopped time. Not only that, but it looks cool, too. It's the first time a character in Dragon Ball can be seen with a double aura, as the red energy of Kaio-ken envelopes Goku's Super Saiyan Blue energy.
It's a whole new level of power for the character, but the drawback could be severe. Back in Dragon Ball Z, simply using Kaio-ken x4 for an extended time wrecked Goku's body. Expect similar results here.
Dragon Ball Super continues to hold surprises for longtime fans of the franchise. It remains unclear if Goku's gamble will pay off, but fans should have a great time finding out.