The following contains spoilers for My Hero Academia Chapter 351, "Two Flashfires" by Kohei Horikoshi, Caleb Cook and John Hunt, available now in English from Viz Media.
Chapter 351 of My Hero Academia pitted Todoroki Shoto against his demented, long lost older brother in his toughest match yet. Since he believes Dabi isn't past redemption yet, Shoto hasn't taken the offensive and is attempting to reason with the villain. Unfortunately his efforts are falling on deaf ears and Dabi is doing his utmost to kill Shoto. Interestingly, Dabi's maximum firepower has no effect on Shoto as the young hero is easily neutralizing his brother's fierce flames. In the end, he finally debuted his version of the Flashfire Fist.
Flashfire Fist is the boys' father, Endeavor's ultimate technique. To minimize his Quirk's effects on himself while maximizing his flames' intensity, he builds up as much heat as he can contain in a single part of his body. It gets released as a powerful, concentrated beam that incinerates anything in its path and leaves him weak when he uses it more than once in quick succession. It wasn't a move he could use lightly -- and that made it all the more shocking when Dabi began his attack with the Hell Spider variation of the Flashfire Fist.
Shoto was spared the brunt of Dabi's first attack by the heroes supporting him. The "Flaming Sidekickers", Burnin, Kido and Onima, redirected Dabi's Plus Ultra Hell Spider, allowing Shoto a few more seconds to charge up his new super move. Unfortunately, their caution was exactly what Dabi expected as the Hell Spider was only a distraction to prevent them from realizing he'd closed the distance to Shoto.
Dabi immediately began to pummel his brother with flaming fists, all the while exuding white-hot flames from his whole body. Before engaging Shoto, he'd already raised his body temperature higher than even Endeavor's maximum output, and Dabi only continued to burn hotter as he verbally and physically assaulted Shoto. Finally, Dabi released all the heat he'd been building up in an ultimate attack -- his version of Endeavor's Jet Burn -- that shot condensed flame from his entire body and launched Shoto several feet in the air before he violently crashed into a building.
During one of Class 1-A's training sessions, Shoto claimed to have almost perfected a body that "even Dabi couldn't burn". Evidently, he completed that training as he was surviving temperatures that exceeded those of Endeavor's attacks during his battle with the High-End Nomu. Endeavor's heat would have decided the match in a single hit without the Nomu's regeneration, yet Shoto handled all of Dabi's attacks without issue. After numerous grueling sessions training both his ice and fire Quirks, Shoto was able to neutralize Dabi's heat without breaking a sweat.
As soon as he emerged from the building's ruins, Shoto finally let loose the super move he'd been preparing throughout their altercation. He called it the Flashfire Fist: Phosphor and mentioned it was a move he'd created specifically to defeat Dabi. Using both his right and left sides in tandem, Shoto bathed his brother in a sea of ice and fire, effectively pinning him down. Even more impressive, he'd finally succeeded in creating flames with his right side.
With this feat, My Hero Academia has signified Shoto's complete mastery over his Quirk. Phosphor's coordinated ice and fire attack might not be enough to completely defeat Dabi, but the momentary edge it gave Shoto could be enough to turn the battle in his favor. And since both sides' temperatures cancel each other out, there's no danger of frostbite or overheating; Shoto can probably use Phosphor as many times as he wishes.