Gonna have to assume you are American here.
looking at the pure facts. They simply DO have the power to massively tank the US economy and cause a huge recession there. What is sad is that Americans like you ignore the facts and massive ever growing debt, saying "But America will win no matter what reality says!"
Would it hurt China to pull out and cash in debts? Yes, it would but as a country they are entirely self sufficient and have diverse trading partners in Europe and Across Asia. They are also entirely solvent as a country. They don't have debt. Their economy would take a small hit and sales would drop but their economy would easily survive while Americas would not. China Simply does not rely on debt and the world economy like America.
America cannot fund itself. The ONLY reason it currently survives is by borrowing billions a year. Your own president admitted not long ago, that if they didn't raise their debt ceiling yet again to allow them to keep borrowing billions more, that their economy would fail.
If China calls in debts or backs out, America looses around 1.4 trillion. This HAS to be picked up by other creditors like the Fedral reserve of other coutries, but America already owes them trillions. They either find new creditors or they immediately go into a huge recession and cannot fund themselves, taxes must jump by about 15% just to cover national bills. Even if they magically do get all that debt covered, they loose their credit rating and have to pay more for their next years debt with less people willing to buy it, this has been seen before in economies and is a dam fast slope to recession.
Simple economic facts. China CAN tank the American economy and send it into a massive depression spiral by cashing in its debt. Even American economists know this, their entire system is based off borrowing money and huge climbing debt, loosing 1.4 trillion would be a massive blow that would make previous recessions look like nothing.