You say this like it's a bad thing. On the grand scale, devaluing the dollar, in effect, devalues China's national wealth by an equivalent percent. If China collapses the currency, they also collapse their own national reserves.
The domestic effect of devaluing the dollar is that imported products become more expensive, primarily oil, food, natural resources, etc. However, this also means that imported consumer goods are more expensive as well, giving a competitive edge to domestic producers to compete profitably with offshore importers, i.e. manufacturing jobs stay in America. Also, American produced goods can be exported more cheaply, again giving domestic producers an edge in international markets, keeping more jobs in America.
Canada has for years relied on the low value of the Canadian dollar to maintain a competitive advantage in manufacturing cost. The recent decline of the American dollar makes it much more attractive to move production back to the U.S. from Canada. Same goes for China. An over-valued currency is partly responsible for the vast number of manufacturing jobs that have been shipped overseas. A properly valued currency should see some of those jobs return or at least stem the losses.
