Can someone here explain what this guy is alluding to? -- (Implications of Chinese sustained denial of currency appreciation)

In this video:

http://www.youtube.com/watch?v=iC-ulGJ-tLU

The speaker talks about problems with the American economy as if they are anchored around the idea that 'free market capitalism' only works of no significant powers are manipulating the local economy as to artificially impact the global economy. He claims that China's sustained denial of currency appreciation has somehow translated into a benefit for China and a massive detriment to America.

Can someone here with some intl. economics knowledge either reinforce or debunk this man's claims?

I don't understand the implications of a powerful economic player choosing to fix the value of their currency.