Each stock is unique to their own levels. Its neat how a stock reacts to an imaginary line, but they do it.
Think of a football team trying to convert a third down. They've historically had trouble doing it, but their offense (buys) is now rested, and the defense (sells) is now tired. They make the conversion and the cycle starts all over again. If they fail on the conversion, they play defense until they get the ball back. But once the offense has strong momentum running (volume), the power is a lot stronger and can break resistance over and over.