I suspect what is meant is that China might sell a whole bunch of US bonds. It is an interesting question, but is probably not something that is likely to happen due to the dependence of China on a healthy US economy.
But what would happen? If no action is taken by the US then the expectation would be that US bond prices would collapse. In other words US interest rates would increase (possibly dramatically) and if interest rates increased sufficiently, given the amount of private debt many could be overwhelmed by interest payment burden. It wouldn't be pretty!
So faced with that, the FED would simply purchase the bonds using QE money printing thereby extinguishing the debt. Nothing has changed - imports/exports still happen so no reason for exchange rates to change - except that the government is no longer needing to make interest payments, let alone payment of the principal.
