Quote Originally Posted by Zeromus_X
When you're at critical health (HP numbers are yellow or red), there's a chance that character can use their Limit Break. You'll hear a sound and the Fight Command will have an arrow next to it. Press right on the Fight command to access the Special Attack.

You can also achieve the same thing with the spell 'Aura', although it doesn't have the same benefits of having critical health.

Also, you may have to try more than once before it appears. Keep pressing Triangle (or whatever button you use to switch characters' commands in battle) until it pops up.
That's totally not sarcasm.