The ability to use the bellows to presumably find out, in a succinct fashion, what Westley wants to return for suggests that in every story, there's one small nugget of truth that guides the overwhelming logic of the story. When Max goes straight for this after deciding that Inigo is lying, it shows that he's already learned that people will try to say all sorts of things to get what they want, but he suggests that the truth will prevail in the end.