Something specific that was interesting about the display, apart from their speed and intensity, was their use of the centre backs to expose Athletic's man marking. One of the problems teams have faced against Athletic this season has been the intensity of their man marking and their high pressing game, most evident against Man United over two legs in the Europa League. It is a very demanding system and has been hard for Bielsa's team to consistently perform to such a high level but when at it's best, has been difficult for opposition teams to defeat. One of the first times this was evident was in the 2-2 against Barcelona in November where, for much of the game, Athletic made it very difficult for Barcelona.
However in this game, Guardiola's side intelligently used both the centre backs to expose it. One of the things about Athletic's pressing system is that they often use only one player (Llorente) to press against the two centre backs. This doesn't happen all the time and they do use their wide men to press inside on occasions but from a general perspective it is often left 2v1 in this area. This means that one of the centre backs is left free when the other invites pressure.
What Barcelona did, (and also did in the second league game in March where they won 2-0) was to allow the free centre back to move into midfield on the ball and create a situation to allow the midfielders space to get away from the man marking of Athletic.
This is an image seven minutes in.
Gerard Pique, the right-sided centre back moves forward with the ball. Meanwhile Athletic man mark these areas to stop the easy pass and to make sure the midfielders can't turn if they receive the ball.
However Pique, under late pressure, keeps going forward with the ball. In this situation the midfielder marking Xavi's movement is being run at and has a decision to make as to whether he keeps marking Xavi or goes to close down on Pique, leaving Xavi free.
In this situation, he ends up having to go and pressure Pique, leaving Xavi free as a spare man in the middle.
Though the three goals did not come from movement like this, dangerous situations were created as a result of using this to allow the midfielders to get free from the marking.
This was well highlighted by Rafa Benitez on the Sky Sports coverage, in this situation 22 minutes in:
You can again see the midfield marking from Athletic. This time it is Pique again who is moving forward with the ball into the central midfield zone.
Again, the midfielder (De Marcos) marking Xavi is left with the quandary of sticking with the marking or going to pressure Pique, which he does again, leaving Xavi free and very importantly facing the play, allowing Barcelona to dominate on the ball and to try to find the penetrating pass.
Next we see the other centre back Mascherano moving forward in possession just a few seconds after the previous situation.
Mascherano picks up the ball and begins moving forward into the midfield area.
This time he runs at the opponent marking Iniesta. The Spaniard himself moves deeper and to the side as Mascherano gets closer, giving himself the space to be able to turn on the ball and face play ahead of him
As he receives the ball, the opponent who was marking Xavi in the original shot then goes to close him down, leaving a big amount of space between the lines.
As Iniesta beats him, Athletic now have three players (Iniesta, Xavi and Messi) free in front of the defence, not a good situation for any team. This highlights the difficulty of pressing Barcelona - you either press high and try and stop their midfielders turning to face the play, or you allow their midfielders time to face play but make space compact in the final third, making it difficult to penetrate through. The first idea is what Athletic tried to do but with the use of centre back moving forward on the ball and running at players, it allowed the midfield to get free of their marking and to get space to turn.
Here in another situation in the 31st minute of the game, Barcelona's midfield again highlight their clever use of movement to get space.
Mascherano has the ball at his feet. Busquets, the closest midfielder to the Argentine, then moves forward dragging his marker Ander Herrera away, and leaves Xavi the room to move into, marked by De Marcos.
As the ball is switched to Pique, Xavi is tracked by De Marcos tightly. Again though, Pique moves forward and commits the midfielder, allowing Xavi the space to receive the ball and then dictate possession.
In many ways this isn't necessarily the fault of a man marking system in midfield; even with zonal systems, tracking and man marking plays a part in several areas. But here Barcelona demonstrated, with use of the spare centre back, that it is very hard to stop talented midfielders turning to face the play and also that centre backs who are comfortable in possession are very important assets of a proactive passing side.