Break occurs when a character's bravery points are completely depleted by the use of Brave attacks or Summons. When this occurs, the character will have to wait for their bravery points to recharge over time, or use Brave attacks to recharge the lost points. During this time, HP attacks will deal no damage whatsoever, and will only begin to inflict HP once the bravery points have reached the base value. However, Brave attacks will have a higher chance at inflicting critical hits, boosting the bravery points gained. If you are successful in inflicting Break on an opponent, you will be rewarded with their bravery points, plus the stage bravery. However, the opponent can do the same.
All Brave attacks can inflict Break, as can HP attacks (albeit much harder, unless the HP attack deals a significant amount of Brave damage before the HP damage). Certain summons can also influence Break. Odin and Ultima Weapon can inflict instant Break on an enemy. Odin has a 50% chance of doing so, while Ultima Weapon inflicts Break after a set period of time. Golbez can also inflict instant Break during Ex-Mode, by pressing and holding R + Square to charge his Black Fang spell. It requires some charge up, but if uninterrupted, the foe will be inflicted with Break, and you will be rewarded with stage bravery.