Well, back in the day and even in quite a few 2D games today, when something flashes it means that whatever is flashing is about to reach it's end. When you would hit major enemies, they would start to flash indicating that they're about to die, once you had landed the final blow they would start flashing really fast and then explode. I vaguely remember the screen flashing when I beat a boss in Contra, I'm sure that a lot of games used the "flashy screen" technique to say "hey, you won!".

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