Hiyori manages to save Yato from the underworld by discovering that his true name was Yaboku. How did she come to this conclusion?
Since calling him by his name "Yato" didn't work, Hiyori knows that name is fake. Yet, she recalls how fondly Yato looked as his small shrine and the name engraved on it. So she deduces the name engraved in the shrine is correct, but pronounced differently.
She changes the reading of the katakana ト(to) to the reading of the very similar kanji 卜(boku).