It’s the final test before approval for the Final Selection, but it seems impossible. How did Tanjiro slice the boulder?
Tanjiro was able to slice the boulder after training with Urokodaki’s former students. Makomo teaches Total Concentration Breathing and Tanjiro applies it against Sabito, successfully striking his mask. It’s revealed that a ghost-like Sabito stood in the boulder’s place, and Tanjiro successfully sliced through.
Sabito and Makomo are revealed to be ghost-like figures, having passed away during the Final Selection. They offer aid to Tanjiro in hopes he can defeat the demon who has slain so many of Urokodaki’s students. It’s debated if Sabito would be stronger than Giyu, had he lived.
Tanjiro learns how to slice the boulder
In chapter #5, Tanjiro pitifully attempts to slice the boulder. It’s the last test before he can receive approval from Master Urokodaki to attempt the Final Selection. Sabito, a previous student of Urokodaki who claims to have cut the boulder, begins sparring with Tanjiro.
Outclassed and outskilled, Sabito knocks Tanjiro unconscious after a brief sparring session. Sabito only uses a wooden sword against Tanjiro.
Makomo, another previous student of Urokodaki, offers to help Tanjiro refine his skills after each sword-session with Sabito. She helps him point out the defects in his style, as well as correct unnecessary movements and bad habits.
She teaches Tanjiro the important Total Centration Breathing technique for the first time. This technique accelerates the users’ bloodflow and heartbeat, increasing strength to that of a demon.
Tanjiro utilizes the technique over the course of 6 months. He constantly attempts to defeat Sabito in a sparring match. Yes, Sabito’s wooden sword routinely knocks back Tanjiro. Once the half year passes, Sabito approaches Tanjiro with a real Nichirin blade.
The Total Concentration Breathing pays off. Tanjiro is able to slice Sabito’s mask before the enemy’s blade reaches. The cracked mask falls down and Sabito smiles, fading away in the mist to reveal a sliced boulder where he stood.
So that’s the answer. Tanjiro is able to slice the boulder because he trains with Urokodaki’s former students. Makomo teaches Tanjiro techniques and he applies it against Sabito. Once he gains the ability to defeat Sabito in combat, he actually strikes a ghost-like figure who stood in the place of the boulder. Tanjiro slices through that rock instead of Sabito.