How was it SHIKI saved Shiki?
From the novel, the doctor said that Shiki jumped the instant before the collision, so there wasn't much physical injury. The head experienced some collision thus put her into a coma. Aozaki also states later that SHIKI died spiritually.

From the narrative it seemed that SHIKI died during the collision.

At the end of the chapter, Shiki herself states that SHIKI probably died in a suicidal sense, because he wanted to avoid himself harming Kokutō.