What they want is a very slowly appreciating Yuan. This shows that the economy is growing, and it gives China more buying power overseas, but it keeps imports affordable. This is what has happened over the past 10 years, since the government allowed the exchange rate to float. (see https://www.google.com/finance?q=CURRENCY%3ACNY&ei=bD_fVZnNN...).
The issue is that over the past month, the Yuan has actually started to DECREASE in value against the dollar, which is a sign that the economy is slowing. That's scaring the crap out of China investors, who have baked in a very high growth rate into their pricing. That's what's driving the government to put their rate control machine into reverse -- instead of trying to keep the rate from growing too fast (to favor exports), they are doing what they can to keep the rate from falling any further.
They "control" it by using free market tools: buying and selling yuan in the open market.
> China sells the dollars it raised to buy Yuan.
The Yuan is a highly controlled currency that does not behave like other normal free market, floating currencies.
The Chinese government sets the exchange rate as was shown just weeks ago when they devalued the Yuan and Trump call that move the start of a China/USA currency war.
> Demand for the Yuan increases, hence the Yuan becomes more valuable
If the Yuan was free floating that might be true, but even then, since China is a net exporting nation they want a low Yuan, so that their exports are cheaper and their imports are more expensive. So why would they want to drive up the Yuan?
I'm really not sure what is going on but everything they are doing (i.e. selling US bonds) goes against what one would expect.
The only thing I can think of, because of the turmoil in China (i.e. the share market shock, property bubbles etc) they are a bit short of cash and rather than print money (which would causes inflation) they are raising money by selling some of their US bonds.