Basically, there's a book that the Doctor realized is describing the situation with the Angels. The Doctor knows that as long as something's written, it's set in stone, and Amy's read that River breaks her wrist to get away from the Angel currently holding her. The Doctor refuses to help River get out of the Angel's grip and tells her to do it herself without breaking her wrist. He was a huge douchenozzle and it's the last time the really-should-not-be-husband-and-wife duo are seen onscreen together.