The point where the receiving end (more often female than male) experiences such a high degree of sexual pleasure their mind goes into a state of severe stupor. In this state they would lose the ability to think clearly and register information such as pain. Often those in the "mind-break" state unconsciously make an ahegaokin face. It should also be noted that "mind-breaks" happen most frequently during rape and gang-banging scenes.
-from urban dictionary by Adam~kun