Limit breaks occur when a character has taken damage equal to ten times character level without dropping to 0 or fewer hit points.
If a character drops to 0 or fewer hitpoints, the limit count is reset for that character.
When a character reaches the limit break, they may decide to interrupt initiative to take an automatic action, in which case your character will (typically) perform their limit break maneuver.
For example, a character who has maximum hit points of 30 will reach their limit break at 60 points of damage taken…assuming they don’t drop to 0 first.
If they do, they can perform a Limit Break maneuver, which consists of different effects for different characters.
Players are encouraged to design their own Limit Break, keeping in mind your character’s style and personality.