Quote Originally Posted by Zeromus_X
You get it after an event in the story. Rinoa goes into 'Magic Berserk' mode, her Magic stat becomes 255, and she randomly casts spells in her inventory without using up her stock. (So you could give her one spell, like Meteor, and she'll cast that over and over without using it up.)
To expand on that a bit, Rinoa can carry the following para-magic without casting them during Angel Wing mode:

CURATIVE: Cure, Cura, Curaga, Esuna, Life, Full-life, Regen.
SUPPORT: Aura, Double, Float, Haste, Protect, Reflect, Shell, Triple.

So, you junction those, + meteor, to your stats and once in Angel Wing mode, she'll cast Meteor every turn, usually doing well in excess of 50,000 damage per round. Whilst Zell's Armageddon Fist combo can do more damage, it's not as consistent (since the amount of time you have per Duel varies + it's annoying as hell to do), and Squall's Lionheart won't come up everytime, so it's my favourite limit break. When you get those three all blasting away at Omega Weapon, he doesn't seem all that tough.