Now Kefka in that battle has 3000 HP and I would say I was about 1/3 of the way through the battle when Locke out of nowhere pulls this move called Mirage Dive. I know the name because it showed it at the top.
Locke had been in critical for a while before it so I am guessing it had something to do with that. Luckily it dealed around 1800 damage and finished the battle quickly for me.
Is it just a last resort move the characters do by themselves or what?
Yeah, pretty much. As many times as I’ve played through FFVI, I’ve only seen one of those pseudo-limit breaks once. Coincidentally, it was Locke’s, but it was during the TunnelArmr fight. Y’see, I used to be incredibly miserly with my items, especially that early in the game, so I’d hold back the Thunder Rod "for an emergency," even though I knew I’d find a bajillion of them later. I’ve since gotten sick of that Runic tutorial and now go for the Rod right away, turn one kill.