[CE]- US vs China

I was looking through Current Events and I found this article, and this

An angry China warned Washington on Tuesday that passage of a bill aimed at forcing Beijing to let its currency rise could lead to a trade war between the world’s top two economies.

     China warns the United States of a possible trade war if they go through with the bill allowing china's curruncy, the yuan,  to rise in value. Apparently China has been under valueing their currency, the yuen is actually 15to 40 percent below its actually value.They keep their the value of the yuen low by buying US dollars in the open market. By buying dollars off the market they keep the demand of the dollars high. Thus keeping the value of the dollar high, and the value of the yuen low.
     So the question here is why would china want to undervalue their currency? Well first of all to be fair to china, almost all countries undervalue or manipulate their currency. Anyways apparently china likes to keep the value of their yuen low because this way it helps exporters pocket more money. For example, (this is the example from another article) If one dollar buys 7 yuans and an exporter sells a shirt from china for ten dollars he would pocket 70 yuans, while if one dollar was to buy five yuans, the exporter would only pocket 50 yuans. Therefore it is obvious why china does not want their value in the yuen going up. If the United States would allow the yuen to increase in value the exporters would not pocket as much money, which would probably mess with their economy.