Barca have been pressing that way for years. It's a great tactic but requires the entire team to be switched on mentally.
They press tightly for the first 10 minutes of each half in order to unsettle the opposition on the ball and stop them passing it around the way they normally would. It forces you into the safer, less valuable passes - essentially turns the whole team into Scott Parker. It's especially effective for them because they have a front 3 that the whole world knows can score in a flash if you lose the ball in the wrong place.
Once the opposition is unsettled they sit back a little more and expend less energy on the press. If they see the opposition becoming more expansive then they press again, but only with one or two players. Usually that's enough for the other team to feel pressured again and slip back into low value passes. If not, then they heavy press again.
It wasn't Barca tiring or changing up tactics, that's just what they do to everyone. What they didn't count on was the ferocity (and bravery to put it in Poch terms) with which our players came out fighting. Lamela and Kane especially were scrapping for everything and we were actually leaving their front 3 against 3 or even 2 of our own at times and relying on the fact that they wouldn't play it long.
