Both arms dangling

I was a little confused with why if he moves both hands up, stops then disengages why this was a balk. But the reason as explained to me is if the runner breaks when he moves both hands, as he should, but the pitcher stops. If he tries to get off the rubber to make the play then he's gained a tremendous advantage.
So if he moves both hands he has forfeited the right to disengage. You are correct that isn't exactly the way the Fed rule is but it seems to work.