I learned both and it depends on the context. If you can see the other car and they can see you, flashing the lights means giving them right of way or showing them that they can go. If you are coming towards a blind corner it is just a way of bringing attention to you coming. I also do this when cars with high beams come over a crest. Flash your own high beams a few times and they can turn off their lights before coming over the crest blinding you.