It’s so easy to spot the posters who have never played the game to any sort of half-useful level!
There are a number of factors involved for a team to be able to successfully play a high-line
1) The press from the forwards and the midfield has to be spot on. A Premier League standard player, given a yard of space, can put the ball on a sixpence. If your CBs are on the halfway line and there’s no pressure on the ball, you’re asking for serious trouble!
2) The CBs also need to be aggressive and proactive. Again, Premier League standard players are so good that if you allow a forward player to drop-off you as a CB, pick the ball up and turn against a high-line you’re in big trouble. This means that the CB has to be prepared to go into uncomfortable areas with the opposition forwards and trust that their teammates will cover them. This is partly why teams favour split forwards or inside/wide forwards against a high-press - because it makes the attackers difficult to pick up ans allows them to make diagonal runs in behind (as opposed to straight runs which are very hard to time and hard to pick out for the passer)
3) Contrary to popular opinion, you don’t need two fast CBs. This is a false comfort because let me tell you, if Jamie Vardy or Marcus Rashford or Son or Gareth Bale get a run on a CB, that CB is toast, whoever they are. The build of a CB compared to a forward added to the fact the forward has the momentum advantage (i.e. they are running onto the ball, not turning and chasing) means that the CB has virtually no chance in a foot race if the pass is right. Far more important than just having quick CBs is getting the press right and getting the line right...i.e. knowing when to squeeze and when to drop-off
4) You absolutely need a GK who will cover the space in behind. The opposition will look to turn your defenders and knock balls over the top but it your line is right and the press is right, it should be almost impossible to land a straight pass on a modern pitch between the CBs and the GK, if you have got your positioning right. When I say “modern pitch”, what I mean is that the ball will skid off the surface of the pitch naturally, because they are so slick and pristine. This means that successful passes in-behind are likely to have to be angled, and this makes it harder for the opposition to work the right opportunity to hit the right pass (see Villa vs Liverpool for an example of how to execute it well)
5) The CBs have to be really good and really brave! Never underestimate the skill required to play high. Every CB up and down the country from Sunday League to Premier League would LOVE to sit and defend their own penalty box for 90-mins if their manager would allow it. Sure, life would be harder for their teammates but for the defenders, it would be a dream. No space in-behind, everything happening in-front of them, lots of bodies between opponents and the goal...even an average clogger can look good in this set-up. To play high requires a totally different skillset, including immense communication, concentration, teamwork and aggression. It’s one of the main reason people bemoan the idea that defenders are getting worse, which is the biggest load of rubbish I have ever heard. It’s purely that the best CBs now play virtually on the halfway line, which would have been considered crazy in the Nineties/Noughties.