Unlike previous games apart of the Yakuza Series, Yakuza 6 includes the newest Combat feature of Extreme Heat Mode. After earning at least one Heat Orb during battle with Kiryu, you can activate Extreme Heat Mode by clicking the R2 Button. When in his Extreme Heat Mode, the attack damage and speed of which Kiryu can attack increases exponentially. This state doesn't last all that long, but while it's active, Kiryu can unleash loads of incredibly powerful attacks against his enemies.
On top of this, Extreme Heat Mode allows Kiryu to use a number of different moves in general. In order to learn how to use these different attacks, you view the prerequisites for each of these moves by going into the Heat Tab apart of your Stats Menu. While in Extreme Heat Mode, Kiryu also won't be stunned by any attacks landed to him. You will even be able to consecutive punch your opponent simply by repeatedly clicking the Square Button.
As you might guess, practically everything Kiryu does during Extreme Heat Mode causes massive damage to foes. (His attacks can even interrupt the attacks of his opponents!) Sometimes it is best to save your Heat Orbs in order to use your Heat Actions later on, but should you ever be surrounded by a number of enemies, definitely think about going into Extreme Heat Mode to ward them off since most of Kiryu's attacks will hit multiple targets at once.