Field Break is a mechanic that destroys the Magnetic Field around a MAD to decrease their Action Count and allow for added damage via Pursuit when using Special Skills or Harmonics Chains.
MAD deploy a Magnetic Field (MF) to protect themselves. When you view a MAD's information, the bar labeled Magnetic Field is the MF's Durability Value turned into a graph. The MF's Durability Value decreases with attacks, and when it becomes 0, the MAD enters Field Break status.
Causing a Field Break will decrease the MAD's Action Count by 1. When a MAD's Action Count is at 1, it may change its mind about Targets or Skill usage. Also, performing a team attack like a Special Skill or a Harmonics Chain will activate Pursuit and deal added damage.
The MF value recovers to its maximum value during each of the MAD's turns. Because you will have to cause another Field Break after it recovers, it's important to consider strategic timing when causing a Field Break.