Despite Bayern having more possession of the ball, which is generally their style - Barcelona are very confident and in control on the pitch. A quick goal and afterwards a confident dominance in the scoreline shows that Barcelona are very good at the moment and it seems that they have broken a series of poor performances against Bayern. Rafinha's hat-trick I think will be remembered for a long time.
It cannot be denied that Bayern are very good at controlling the ball and the rhythm they do is also actually quite good, it's just that in this case Flick knows Bayern's weakness is the fast counter-attack carried out so that even though Bayern managed to have possession of the ball, Barcelona's momentum and rhythm were much better in this match.
The 4 goals scored by Barcelona in this match all started from quick counter-attacks and actually if we look at almost of these 4 goals occurred because of the mistakes of the Bayern players both in the offside trap and the anticipation they failed to do which ultimately led to goals.
The first goal, occurred because of the players' mistakes due to the failed offside trap, the second goal occurred because the Bayern defender failed to intercept the ball, the 3rd and 4th goals were almost the same as the first goal process so in this I think Flick really explored and read the Bayern game which made them lose and ended their dominance over Barcelona over the past few seasons.