If the president of Brazil, Lula, decides to increase import taxes, the US will mirror this decision. As a result, Lula has set a tax rate that US applies on imports and that US buyers end up paying to the US government.
Yes, they have just decided not to control it and left it in the hands of Brazil.
If they are smart, they just won't follow on the promise... but they haven't being doing that kind of thing recently.
Anyway, we will probably not see any trolling, because increasing the tariffs from Brazil's side would be a minor economic setback, and there are elections next year.
No, while you mirror, you are not in control. But sure, you can decide to stop the mirroring that you started even though now it puts you in a worse situation than where you started. Say, Lula sets the import tax at 100% - will Trump say "that's too high a number for me, I fold"? In that case, he wanted the rates to be the same but now the difference is even higher.