It's exclaimed, most often in a group, when playing a videogame and the screen suddenly starts to flash with vibrant colors or in black and white. An example would be in MegaMan 2 at Wily's Castle, every time you destoy a boss, the black background starts to flash vividly and quickly in black and white. It's seen more if the game is played in a dark room, in which the whole room is lit up.
1. *boss defeated, screen starts to flash* *whole group* Seizure!!!