China needs to buy dollars to backstop their own currency (¥uan). When China trades with the U.S. they hold the return dollars as a peg against their weak currency.  Remove the flow of dollars (lessen exports) and they start to run out of strong pegged currency.
What is happening today is not as much direct devaluation by China; rather they are intentionally allowing their currency to drop in value, in an effort to lower export prices and off-set any tariffs from the U.S.   Simultaneously, Beijing is spending internally, burning cash, to keep their economy from weakening.  Their Yuan burn rate is greater than the influx of higher valued dollars needed to hold their position.
They cannot keep this position indefinitely.
First, here’s a solid interview with former CEO Gerald Storch on how the currency devaluation leads to lower prices for U.S. consumers.  Again, emphasizing the point that U.S. consumers are not paying for the tariffs against China.  Watch:

President Trump responds to the current activity by China:

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Beijing says they will keep lowering the value of their currency to keep the flow of cheap Chinese manufactured goods flowing to the United States, regardless of tariffs.  Simultaneously they need the returning higher valued dollars to prop-up their own diminishing currency.

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Hang on folks…. this is going to continue: