It’s an unexpected move, but Tanjiro is able to harm the Wind Hashira with his arms behind his back. How did Tanjiro headbutt Sanemi?
Tanjiro was able to headbutt Sanemi by launching himself into the air. Despite his hands being tied behind his back, Tanjiro is still able to push off his backfoot and collide with Sanemi.
The arrival of The Master interrupts Tanjiro and Sanemi before the fight breaks out into something worse.
Tanjiro meets the Hashira, is forced to headbutt Sanemi
In chapter #45, Tanjiro awakens at the Demon Slayer Headquarters. His arms are tied behind his back and he’s pinned to the ground. All of the Hashira stand before him, sharing their opinions on how to handle Tanjiro and his demon sister. The Hashira, albeit with different emotions, unanimously agree that Nezuko can not be left alive.
Tanjiro thinks of Nezuko and his friends, panicking when he doesn’t immediately recognize their locations. The Snake Hashira mentions that a punishment must be served for Giyu. The Water Hashira has tolerated Nezuko’s existence up to this point.
Pleading his case, Tanjiro explains to the Hashira that Nezuko has not eaten anyone in the two years she’s been a demon. He says he only became a Demon Slayer in order to protect her and find clues towards restoring her humanity.
The Hashira hardly side with Tanjiro. They view Nezuko’s case as open-and-closed. Within their rules, no unverified demon is permitted to live, even if they exhibit helpful behavior in combat.
The Wind Hashira attacks Nezuko
Sanemi Shinazugawa, the Wind Hashira, approaches the group with Nezuko’s box in his hand. He aims to prove that Nezuko is incapable of controlling her bloodlust and fighting alongside the Demon Slayers. Samemi thrusts his sword into the box.
“I don’t care if you’re a Hashira or whatever,” Tanjiro says. “I won’t let anyone hurt my sister!”
At that moment, Tanjiro pushes his foot off the ground and launches himself headfirst into Sanemi. Not expecting the move, Sanemi’s head makes a complete collision with Tanjiro, knocking them both to the ground.
Tanjiro reprimands Sanemi in frustration. He says that if Sanemi is incapable of telling the difference between a good demon and a bad demon, then a Hashira title is undeserved.
So that’s the answer. Tanjiro was able to headbutt Sanemi by pushing off his back foot and launching himself into the air.