The Yuan buys half what the USD buys inside China, so the Chicoms keeping the peg at around 7 Yuan to each USD is causing an imbalance that labor arbitrage can make alot of wealth on.
Say you take $1000 and convert it into 7000 Yuan. You then spend it on commodities so you can then make 700 widgets in China at a cost of 10 Yuan each. That 10 Yuan made $5 worth of widgets, and then you sell the widgets at cost and have $3500 to pocket. You take that $3500 and repeat the cycle making money hand over fist.
The key is that the Chinese will sell you more stuff than your dollars are actually worth. Japanese have done this for years.
That is why Wallmart is so successful; they are playing the labor arbitrage game that the Chicoms make possible by undervaluing the Yuan by a 2:7 ratio.