China is creating controlled inflation - they are basically printing new yuans. That might seem like a bad idea, but China is using newly created currency to buy US treasury bonds (actually buying US debt). China is basically creating a huge leverage on US and on the same time it gives USA cheap debt to buy it's own products, and weak Yuan also causes their exports to be even cheaper.

On May 2019 china had 1.11 trillion USD bonds. And at any point china may decide to sell it off - which will crash USD.