One Piece: Burning Blood is a three-on-three fighting game, meaning you’ll have two characters in your corner at the start of a match. You can switch between your characters using the triggers on your controller, but they have other uses as well.
One such use is called the Unity Assist. This technique breaks an incoming attack and calls your selected teammate in to do a powerful, unblockable attack. To pull this off, though, requires three segments of your Burning Gauge. In exchange, you can turn a fight around if you find yourself trapped in a corner getting obliterated.
When you have three bars full, and you have at least one teammate alive, press the Swap button while being attacked. This will consume the entire Burning Gauge as well as switch to a cutscene of your partner coming to save your hide. After they pull off their special ability you will regain control and can capitalize on the stunned, injured opponent in front of you.
You should only use this technique if you find yourself in a bind. Using it when you have three healthy fighters is not the best way to make use of your meter during Burning Blood fights. It’s also a good idea to try to force opponents to use it by keeping the pressure up with a fast-paced, in-your-face type of fighter like Luffy.