How do shifts the reader understand the message of the story? !

Not completely sure but I think that shifts give clues to the reader (most likely through devices such as imagery and emotional context). Shift creates foreshadowing and suspense, it allows for there to be situational irony in the story, instead of blindly leading the reader on. Therefore, shift gives the readers insight to the story, creates foreshadowing, suspense, and situational irony (which makes a great book) so that the book isn't just blind details leading the reader through the dark the whole time (if this was the case, then it would not be a desirable book).