In MK vs. DCU, the Rage meter is broken up into halves with Breakers using one half of the meter to perform. In Mortal Kombat (2011), the meter is composed of thirds, with a Breaker requiring two thirds of a full bar to perform. Thus decreasing the limit in which they can be done in those games.
Breakers were created when players could perform sequences of uninterruptable combos in the earlier games. As a result, Breakers were added to minimize the damage.
The effectiveness of the Breaker is debatable. While they may save the user from very damaging combos, they share the same power up bars as other unbreakable attacks like Rage from MK vs. DCU or X-Ray Attacks in Mortal Kombat (2011).