Break is an effect that occurs when attacking limbs with blunt weapons or fists. When you break an enemy's bone, the limb becomes useless and the enemy can't move it anymore. Breaking ONLY works with zombie enemies.
Broken Head/Neck: Causes zombie to roar in pain but does not affect its combat, although it's good for buying time to regenerate stamina.
Broken Arm: Causes zombie to attack with opposite arm. In the case of an Infected, it instantly dies by falling to the ground while bleeding out. If a zombie is lying on the ground, breaking both its arms will instantly kill it, no matter how much health it has remaining, due to its inability to regain its footing with two broken arms.
Broken Leg: Can only be broken while zombie is knocked down, causing it to limp when it gets up. Also, if you aim for a zombies' leg and break it, it may not be able to get up.