China informed the US that its going to be selling its treasury notes (for dollars) so that that it can buy back renminbi (its currency) in dollars to increase the value of the renminbi versus the dollar. With any other asset this would be a sort of 'who cares', however the US finances its own government debt by selling treasuries and having two sellers on the market means that you start competing for buyers, and you compete by selling them at a discount (which raises their effective interest rate).

What that means then is that even though the Federal Reserve is interested in keeping interest rates down, you can see interest rates of treasuries can be pushed up, and for the US, selling new treasuries to finance the deficit will pay more interest (causing the deficit to be that much worse) and that can also push interest rates up.

Bottom line, China did the right thing by letting the US know they are pursuing this strategy in order for the Federal Reserve to continue to manage its own economic policy.