You all miss the point. The HK dollar is a convertible currency, acceptable world-wide. That means if the HK government wants to peg the HK dollar to the US dollar, if valuations change on the international currency markets, the HK government must buy or sell dollars/ HK dollars on the open market to keep it's value at the current level. Sometimes, that can cost the the domestic monetary authority a very substantial sum, but they carry out this policy because they believe its best for financial stability in HK.
OTOH, IN China, where the Yuan is non-convertible outside of China, they do not have to worry about its international value, only its domestic value. Thus controlling the domestic value set by the PBOC only requires that the government raise reserve ratios for domestic banks or increase the issuance of domestic bonds to suck back all the currency that was printed to buy the foreign currency thats coming into China as FDI or as payment for exports. This process is known as sterilization. However, it is far easier to keep a non-convertible currency at a set value because you don't have outside sources having an influence on its value - -either though purchases caused by FDI or exported products, or speculation/investment.