Ok, I watched the video slowly (aka frame by frame). There is no flash or "star" when he presses a key, it's the standard "grey out upon keypress". The flash you are seeing is the already mentioned AddOn, Spellflash
The flash is happening exactly the way Spellflash works:
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This addon is able to make suggestions of what spell you may want to cast next by flashing on top of the spell buttons.
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The flash appears on a button yet the cast bar is triggered for another spell (example flash on Mind Flay, however Mind Blast is being cast)