This gesture is usually used by drivers to indicate the the cars behind you that they should slow down (traffic is slow up ahead).
I've also seen it used once by and old man trying to cross an intersection. it is important to know that in Brazil pedestrians are second-class citizens as far as drivers are concerned. Pedestrians are expected to run out of the way as a car approaches and it is quite common for pedestrians to jump out of way at the very last moment. Problem is if you don't get out the way you might get hit as my Canadian friend got in São Paulo (not seriously hurt). If you can't get out of the way in time use this gesture to give the driver some warning, it may save your life.
