No, this isn't like the three second rule where you can still eat something off the ground if you pick it up quick enough. But yet, in a way it is, because it's all about reacting quickly.
After Barcelona loses the ball the whole team fights to win the ball back in six seconds. Once they lose the ball they don't drop back or concede space, no, they all press the ball and try to win the ball back as soon as possible.
Related to this, is one of their secrets to success - pressing the ball when any member of the opposing team turns their back with the ball. Watch how Messi fights to win the ball back in the attacking third, if he actually loses the ball, rather than letting the other team move up the field and towards Barcelona's goal.