All-Out Attacks are available after you fell all enemies in a combat scene into a Downed state. Instead of choosing to Back Out or Talk, the
button will rush all active members in your party (read: not inflicted with an ability that makes them unusable) behind Akira (MC) to deal damage and attempt to clear the board.
AOAs cannot be performed if Akira is unusable (even Downed) or if Akira is the only member active. This is sometimes useful for clearing everything faster than wasting more SP/time on fighting them (and you get some snazzy finishers).