WARNING: The following contains spoilers for Demon Slayer: Kimetsu no Yaiba Season 2, Episode 12, "Things Are Gonna Get Real Flashy!!," now streaming on Funimation, Hulu and Crunchyroll.
As he's done several times in Demon Slayer: Kimetsu no Yaiba, protagonist Tanjiro Kamado has upped his game when it matters most. The fifth episode of the "Entertainment District" arc picks up as Tanjiro's battle against Daki, Upper-Rank Six of the Twelve Kizuki, continues. After ruining another blade (seriously, Haganezuka is going to kill him), Tanjiro decides that his only option is to lay all his cards on the table by using the Hinokami Kagura, despite the risk of becoming immobilized as a result. Yet surprisingly, Tanjiro is more capable than he's ever been while using the Hinokami Kagura, aka Sun-Breathing.
Before the fighting resumes, Daki helpfully points out that Tanjiro has already chipped his blade after only a minor scuffle. The young Kamado chastises himself as a subpar swordsman for damaging his blade so easily. Finally ready to admit that the Water-Breathing Style he learned from Urokodaki just doesn't suit him, Tanjiro decides that it's time to commit himself to the Hinokami Kagura. In a battle against an Upper-Rank, one of the most powerful demons on the planet, Tanjiro can't afford the slightest mistake, especially with the risk of the Hinokami Kagura leaving him collapsed in a heap and helpless.
After the Mugen Train battle, Tanjiro has been training with the Hinokami Kagura but still struggles to unleash multiple consecutive strikes without crumpling to the ground in agony. However, using Water-Breathing against Daki got him nothing but a chipped sword, so the young demon slayer has precious few options if he wants to win this fight or even survive. Inspired by Kyojuro Rengoku's actions and words, Tanjiro sets his heart ablaze and unleashes the Hinokami Kagura with everything he's got.
Surprisingly, Tanjiro is able to perform multiple strikes while only briefly being immobilized by the pain. Feeling his body temperature rise, Tanjiro flashes back to the Butterfly Mansion infirmary with Kiyo, where it's revealed that he can wield the Hinokami Kagura more effectively if he's running a fever. Although initially caught off guard, Daki still calls Tanjiro's new moves slow and "nothing to write home about" -- except she does appear to be trying slightly harder to kill him now, but to no avail.
Tanjiro has a long history of making extraordinary leaps in strength during the heat of battle, so Daki may be in more danger than she realizes as she lets the "entertaining" fight drag on. The Hinokami Kagura has been devastating on every demon Tanjiro's used it on so far, capable of cutting Rui's webs, killing Enmu and wounding Akaza. Considering the Hinokami Kagura is actually Sun-Breathing, a technique that draws its inspiration from demon-kind's greatest weakness, it's not something that any demon, even an Upper-Rank, should take lightly. After all, even Muzan Kibutsuji, the Original Demon, cannot walk in the sun.
If Tanjiro makes Sun-Breathing his new primary Breathing Style, he might just shoot up the ranks of the Demon Slayer Corps. He could even become a Hashira-level threat to any demon he encounters if he is able to completely master the Hinokami Kagura. As it stands, Tanjiro is struggling to hold his ground, but that he isn't already dead is a testament to his impressive progress with the original and most powerful Breathing Technique. However, only time will tell how long Tanjiro's endurance can keep him alive.