It is in the position of emphasis. Inverting the position moves it from the regular declarative to an emphasized declarative. We most often use inflection to convey the sense, but it would come out to something like: YOU are the man behind the curtain. This can either be thought of with certain forcefulness: A) "Is it me? Is it me?" B)"YES, YOU are the man behind the curtain," or it might even be surprise, loosely: "ah-ha, so YOU were the man behind the curtain."
Not really. I'm not a native speaker, but it seems to me that the use of "tu" when "sei" already implies this is meant to be an emphasis on "tu", i.e. You are the man behind the curtain. As for why it follows the verb, I couldn't tell you; after listening to native-speaking professors for two years of college Italian, the best I can say is that it sounds more "natural" this way...
Nope, sorry, but a declarative statement in Italian should still be "Tu sei" not "Sei tu", because word order determines whether or notsomething is an interrogative even in Italian. And if DL marks you wrong for translating it as an interrogative, free app or not, they are the ones who are in error. The directions clearly state to translate into English, not guess whether this should be a question or not
I can count on one hand (5 fingers) the number of times I've heard the lady robot indicate the interrogative by a rising inflexion!!! I look for the question mark, and if I don't see it, I translate it as a declarative statement. The inverted word order threw me off (sei tu) so I lost a heart :-(