I have the advantge of having learn't to drive here... it's part of the standard teórico learning to decipher the flashes. There are four main instances...
1) The first instance when someone overtaking comes t close so you have raise your foot from the accelarator, three swift furious flashes signal...you f****** stupi w****, you're gonna kill us all (swearing colourfully is also part of standard Spanish driving lessons).
2) When someone ahead of you is about to maneuver invading your space... as you're not in a hurry, still have ten minutes till kids come out and you remember how grateful you were when somene last allowed you to complete an awkward maneuver, you magnanimously slow down and flash once to say... ok, you can pull out... normally they don't trust you are actually letting them out... so you slow down a little more, and flash twice to say.. come on then, don't take all day... if they still don't move, you flash finally once more as you put your foot down to say... please yourself, too late now...
3) The third instance is when you see a familiar car, you flash your lights (just once) to say... wow, long time no see, how are the kids, let's meet up and have a drink soon ...often to find a complete stranger who will automatically slow down as he thinks you are applying the fourth instance
4) The fourth instance... the one I really hate is a random flash when you're doing a nice steady speed, complying with all the regulations, but inevitably results in your foot immediately reducing your speed by at least 20km per hour even if you're below the limit and means, the guardia are round the next bend... I don't agree with this one, they are there to catch all the uninsured drunkards who are speeding, but I still react by instinct.