What I dont understand is why does China artificially manipulat its currency to be weaker.
A great point Bodd that everyone is choosing to ignore. Basically China don't allow their currency to be traded on foreign exchanges, thus in effect having the ability to set their own exchange rates. Devalue the currency at will, covers wage increases and keeps a competitive edge, unfair advantage? yes. The Donald is the only world leader I'm aware of who has dared to speak out about this practice.
Now, what he's also spoken out about, is how Germany has in fact adopted the same process with the Euro. The Euro has artificially lowered the value of the DM while inflating the value of the the Lire, Peseta Drachma etc.
Not only that, EU rules dictate a limit to the trade surplus any member state can run, Germany has run at over double that amount for a very long period. The problem is, for every country that runs a surplus, another must run a deficit, hello southern Europe.