From the Super Mario Wiki, the Mario encyclopedia
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KO (originally known as Super Strike) is a status effect in Mario & Luigi: Bowser's Inside Story and its remake that can only be obtained by Mario and Luigi via a badge effect that involves mixing the Powerful Badge with any of Luigi's badges. When in this state, the damage dealt by either Bro is multiplied to a certain amount (depending on the other badge in use) only on their next respective attacks. The status is indicated by stars moving upward, and Mario and Luigi may also strike a battle pose just before using a Special Attack.
In Mario & Luigi: Bowser's Inside Story + Bowser Jr.'s Journey, the status is now indicated by fire surrounding the powered-up brother, but otherwise functions the same.