Team is playing with short passes. While defending wing backs help to defend and so it is very difficult for the opposition even to create a chance for goal. And one of the defenders (generally the one in the middle) comes forward (not too much) and plays like DM. I conceded a few goals and most of them are from long range efforts or corners.
In the attack wing back comes forward and by the help of two cm (one of them is playing like playmaker) i have 7 players in attack.
In my first season with Barca, in the beggining of the year the left MC was a bit more defensive, but then I increased his creative freedom and he became less defensive. After January Messi scored nearly 50 goals.
In first few matches you might be upset, but after 4 or 5 games the tactic becomes really very impressive .
Here I am giving some pictures from the tactic.