I think he's saying that China has bought up Trillions of US government debt aka Treasury Bills. So if they wanted to "spend" it, they can sell it on the open market which would cause their value to drop or wait for the Bills to mature and not buy more. The problem is that all these T-Bills are denominated in US dollars, so China's wealth is tied directly to the US being an economic powerhouse. Any action that could potentially cause the US to default on its debt or to try and inflate its way out of debt results in China devaluing its own wealth.