The first devaluation is just a political move. Just to test the waters - will the Americans go crazy about them devaluing.
In the second act, they will say - hey, this 1.5% didn't work. Look our industrial production is still falling. We have to adjust to market forces and devalue more. Then comes the 20% or so devaluation. But look, the Yuan has appreciated around 8% the last 5 years, so it will only be a relatively small devaluation. That will be the argument.