I spent some time figuring this out tonight, basically, while in light mode, players can't be hurt by the darkness, but also can't see the affects of the darkness. They can solve the human level of the mystery, but not the supernatural one. The shift to dark mode always brings with it a fresh body, and the threat of having your reality distorted and attack from the manifestations of the darkness. There's a third mode that acts something like the X card that Blubeard uses, where players become the writers of the show and can retcon events as they see fit.