[Fr.dr.k S.d.rst.dt]

To add to pashas list, if you happen to be 3 or more cars running very close. if the car behinds you flashes you its most likely cause he feels he is being held up and wants you to make a move on the car in front sooner rather than later, otherwise he will make a move on you.

that said, all of the flashing for whatever reason is part of the game.
its a tool to use and if it makes the guy you are battling annoyed or makes a mistake because of it, most likely had the desired effect.

sometimes its not about how fast you can drive but if you can drive when you feel pressure.