TLDR: Use a DISCOVER Card or AMEX, they don't engage in these shenanigans.
I personally find this so called feature incredibly annoying. When you
are in a foreign country and want to pay for something and use your Mastercard or Visa,
the terminal will offer to convert the bill into dollars for you. Sounds nice right? NOT!
You are charged a fee for this service, and if the credit card you are using has a FX (Foreign Exchange)
fee, the card issuing bank will charge their FX fee anyway because you are using their card in a Foreign Country.
So you are paying a FX fee twice, once for the DCC and then again because you used the card in a Foreign
Country.
If you are using a card which doesn't charge an FX fee, you are still paying for the DCC.
What makes it worse is that depending on the terminal being used, sometimes it says press
the RED button to avoid the DCC, sometimes it says press the GREEN. They purposely
make it confusing hoping you'll make a mistake and opt into the DCC. I've also been
in restaurants where they'll just opt in to DCC without asking you and you find out either
when they give you the receipt or check your credit card statement.
I try to stick to using DISCOVER or AMEX when in Brasil. If it isn't possible, I make it a point to
tell whomever that I am paying in REAIS and not dollars.
You can find out more information about all of this by searching for DCC (Dynamic Currency Conversion).
I'll now get off my soapbox.