I believe this would provide the answer, althout I’m not %100 sure. Perhaps somebody else can approve. Sorry for the lenghy response
Loading up on dollar-denominated assets was, I believe, the method of which the Chinese used to actually peg the yuan to the dollar. In a floating system of currency, the free market determines the exchange rate. The natural tendency of the market would be to set the exchange rate so that imports equal exports. The market, if allowed to operate, would balance the trade between the two trading nations. There would be no trade deficit.
The Chinese, by buying Government securities, were able to interfere with the above process and prop up the dollar. By controlling the amount of US bonds purchases, they could maintain their desired exchange rate.
Here is an explanation of the process from Milton Friedman in “The Case for Free Trade” http://www.freerepublic.com/focus/f-news/1154295/posts
Milton Friedman:
Consider an extreme case. Suppose that, to begin with, 360 yen equal a dollar. At this exchange rate, the actual rate of exchange for many years, suppose that the Japanese can produce and sell everything for fewer dollars than we can in the United States–TV sets, automobiles, steel, and even soybeans, wheat, milk, and ice cream. If we had free international trade, we would try to buy all our goods from Japan. This would seem to be the extreme horror story of the kind depicted by the defenders of tariffs–we would be flooded with Japanese goods and could sell them nothing.
Before throwing up your hands in horror, carry the analysis one step further. How would we pay the Japanese? We would offer them dollar bills. What would they do with the dollar bills? We have assumed that at 360 yen to the dollar everything is cheaper in Japan, so there is nothing in the U.S. market that they would want to buy. If the Japanese exporters were willing to burn or bury the dollar bills, that would be wonderful for us. We would get all kinds of goods for green pieces of paper that we can produce in great abundance and very cheaply. We would have the most marvelous export industry conceivable.
Of course, the Japanese would not in fact sell us useful goods in order to get useless pieces of paper to bury or burn. Like us, they want to get something real in return for their work. If all goods were cheaper in Japan than in the United States at 360 yen to the dollar, the exporters would try to get rid of their dollars, would try to sell them for 360 yen to the dollar in order to buy the cheaper Japanese goods. But who would be willing to buy the dollars? What is true for the Japanese exporter is true for everyone in Japan. No one will be willing to give 360 yen in exchange for one dollar if 360 yen will buy more of everything in Japan than one dollar will buy in the United States. The exporters, on discovering that no one will buy their dollars at 360 yen, will offer to take fewer yen for a dollar. The price of the dollar in terms of the yen will go down–to 300 yen for a dollar or 250 yen or 200 yen. Put the other way around, it will take more and more dollars to buy a given number of Japanese yen. Japanese goods are priced in yen, so their price in dollars will go up. Conversely, U.S. goods are priced in dollars, so the more dollars the Japanese get for a given number of yen, the cheaper U.S. goods become to the Japanese in terms of yen.
The price of the dollar in terms of yen would fall, until, on the average, the dollar value of goods that the Japanese buy from the United States roughly equaled the dollar value of goods that the United States buys from Japan. At that price everybody who wanted to buy yen for dollars would find someone who was willing to sell him yen for dollars.
The actual situation is, of course, more complicated than this hypothetical example. Many nations, and not merely the United States and Japan, are engaged in trade, and the trade often takes roundabout directions. The Japanese may spend some of the dollars they earn in Brazil, the Brazilians in turn may spend those dollars in Germany, the Germans in the United States, and so on in endless complexity. However, the principle is the same. People, in whatever country, want dollars primarily to buy useful items, not to hoard, and there can be no balance of payments problem so long as the price of the dollar in terms of the yen or the deutsche mark or the franc is determined in a free market by voluntary transactions.
Why then all the furor about the “weakness” of the dollar? Why the repeated foreign exchange crises? The proximate reason is because foreign exchange rates have not been determined in a free market. Government central banks have intervened on a grand scale in order to influence the price of their currencies. In the process they have lost vast sums of their citizens’ money (for the United States, close to two billion dollars from 1973 to early 1979). Even more important, they have prevented this important set of prices from performing its proper function. They have not been able to prevent the basic underlying economic forces from ultimately having their effect on exchange rates but have been able to maintain artificial exchange rates for substantial intervals. The effect has been to prevent gradual adjustment to the underlying forces. Small disturbances have accumulated into large ones, and ultimately there has been a major foreign exchange “crisis.”