China sells US debt treasuries in exchange for dollars.
China sells the dollars it raised to buy Yuan.
Demand for the Yuan increases, hence the Yuan becomes more valuable, hence its exchange rate vis-a-vis the dollar appreciates (it will also appreciate vis-a-vis other currencies).
Dollars are being dumped into the market, so the dollar should decrease in value.
A more valuable Yuan means that China's currency is more expensive, hence its exports become more expensive (and drop) and its imports become cheaper (and rise).
Other people now own US debt, for which they will receive interest payments and a repayment when the debt is due.