Yukio's shots have been ineffective, but it's the only thing he can do right now. And then Rin shoves him off Kuro's back, just barely outside of the billowing mist.

"Nii-san!!" Bad choice, Rin. Yukio's pissed.

He needs to get back into the fight, but there's nothing he can do, and he knows it. Yukio, of course, goes into the mist anyway, limping forward before coming to a dead stop as the pain and darkness hits. His claws dig into his palms and he can't move. He can't do anything. He's just a powerless human.