1. Yes, with "you" the intensity is increased. 2. It's not clear what is actually intensified. I would put it differently. Without "you" the command is neutral. With "you" there is a combination of command and warning, as if to say, "or I'll make you regret it if you don't obey this command". 3. There is yet a third level of intensity. "Don't you dare follow her". "Don't you dar