I read something interesting about his tactic today in the German Bayern forum. Why it worked for Bayern in their first season, was worse in the second and why it's working this good at Barca again.
Basic line is, he wants to force the opponent to play the long balls. Firstly, he baits them with the high defensive line to shoot the ball forward and get these easy behind runs like waving a piece of meat in front of a wolf. At the same time, he tries to close the space in front of the defense line, so they can't play these dangerous through balls from a short distance, which would be much more deadly.
The idea behind is, it's first of all more difficult to play an accurate long ball than a short one. Secondly it's easier for the defender to intercept a slower long ball that is longer in the air and gives the defender more time to react, thirdly you can play your offside trap of course.
But for this to work, you need tactical smart DMs with the right position play to close the gaps in front of the defense and bait your opponent for the long balls. In München, he had Thiago in his first 19/20 season, which worked perfectly well and then when he left for Liverpool, Flick didn't get a similiar successor and their defensive problems started.
Now at barca, he has again tactically trained players with Casado, Pedri, Frenkie who understand his idea. And the result was Bayern and Madrid were both baited into these long balls and offside traps.
Of course it's not a rock solid solution you can't overcome, as we've all seen the close offsides and 1vs1 against Pena yesterday. There is no perfect defense in the first place. You can get trashed with a high, low or medium defensive line.
But Flick likes this style of play, because the upside is it allows you to roll over your opponent having many players standing that high and playing fast.
It falls or stands more or less with smart DMs on how much they can bait and force the opponent into these long balls and backline too of course, because if just one player sleeps it's over like we've seen on Madrid's side where Mendy slept and Lewy was alone in front of Lunin. High risk, high reward
was at the wheel, 
was dancing at the back and we were reliant on
to score our goals
