You mean the power he has?

I've noticed it a few times now and I know it's something that keeps trying to... take him over. Shirou was able to sense that presence in him... as well as another.

I saw the aftermath of him being in that state when he took on three of those Academia guys. He was literally acting like a rabid animal when I got there... me and Gongenzaka had to hold him down until he passed out from it.

It's why I called out to him in his duel with Reiji, to try and calm him down. I saw it happening.