In March 2018, when Christian Titz was appointed manager of Hamburger SV, his first order of business was to change the team’s starting goalkeeper for one more comfortable on the ball. The second was to change people’s whole idea of what a goalkeeper can be.

Instead of staying home in the box, HSV’s goalkeeper, Julian Pollersbeck, was instructed to come way outside his penalty area to play between the centre-backs, spreading the back line wider and giving the team an extra man ahead of the ball.

In this build-up, the goalkeeper effectively functions as a spare centre-back – or a keeper-back, if you will.

Goalkeepers had played the odd pass outside their area before but HSV are part of a distinct recent trend. In the last few years, the keeper-back has spread beyond a few adventurous teams to become a familiar tactic, especially in England.

The concept has become so popular that lately, you can see variations on the keeper-back almost everywhere you look. But what’s the point of it – and is it really a good idea?

One basic problem a build-up has to solve is how to keep the opposing press from trapping play against the sideline.

Against Burnley, Manchester United tried to cut the pitch in half by wedging Rasmus Hojlund between the centre-backs and forcing the build-up to one side. Vincent Kompany could have asked his players to send the ball back to the box to get around the press, but a backward pass was Hojlund’s trigger to press through to the goalkeeper while his team-mates stepped up to squeeze Burnley deep in their own half.

Instead, Burnley goalkeeper James Trafford stayed close to his narrow back line, forming a tight triangle that could zip the ball around Hojlund to a centre-back who had room to dribble into space.

The point of bringing out a keeper-back is usually to help play around the first line of pressure. When that defensive line is just one player, it doesn’t take much.

Along similar lines, Tottenham Hotspur broke Everton’s ‘pendulum press’ — where two forwards take turns stepping to the ball-side centre-back while their partner drops to mark the defensive midfielder — by pushing the goalkeeper Guglielmo Vicario up between two very wide centre-backs.

Each time Dominic Calvert-Lewin tried to press Vicario from one side, Everton’s other forward was forced to stay central to deny a straight-ahead pass to the defensive midfielder. That left Spurs’ left centre-back, Ben Davies, free to stroll right past the first line of pressure.

The basic principle is the same as playing around one forward, but with more width at the back to get around the striker pair. The extra space gives the keeper-back a little more time on the ball to read the press and decide whether to play through the middle or out to a free centre-back.

Most keeper-backs operate like a middle centre-back in a back three, but they can also play in a back four, bumping one centre-back out to a full-back slot or up into midfield.

Liverpool’s Alisson is playing as a keeper-back this season more than anyone has in Premier League history, in part because Trent Alexander-Arnold’s half-back role allows for a natural rotation at the back.

Here’s how it looked against Manchester United:

Shifting from a middle keeper-back in a back three to a keeper-back on one side of a back four ratchets up the risk, but it also pushes an extra player ahead of the ball to give the ‘keeper plenty of ways to play out.

So far we’ve looked at ways a keeper-back can free up a nearby player, but sometimes it’s the goalkeeper who’s left as the spare man with time and space to dictate play.

Spanish tactical darlings Las Palmas love to use Alvaro Valles in a central keeper-back role to bait the press a la Roberto De Zerbi’s Brighton, rolling the ball under his sole or playing short one-twos to draw opponents out and open holes for his team to play through.

Not many sides would be comfortable playing one-touch passes under pressure with their goalkeeper so exposed. When it’s done this crisply, though, fast passing with eleven outfield players can make a team practically impossible to defend.

You might be surprised to learn that Brighton – typically early adopters in the space, and a team who led the Premier League in keeper-back actions under Graham Potter – don’t actually do it that much under De Zerbi. The explanation has to do with the trade-offs of pushing the goalkeeper forward.

A goalkeeper stepping up to join the back line creates advantages ahead of the ball but also some disadvantages on the ball. Unlike a centre-back, a keeper-back has no safe passing outlet behind him and can’t play risky forward passes without potentially disastrous consequences. It’s a great tactic for swinging the ball around the first line of pressure but not so hot for building through the middle.

A ‘keeper who stays in the box, on the other hand, is less useful for playing around opponents but better for playing through them. Passing all the way back to goal is inherently risky, but to exploit that risk an opponent has to commit players forward and stretch out vertically, opening space between the lines.

If the goal of your build-up isn’t to push the defence deeper but to break through it with third-man combinations in the middle, those vertical gaps are exactly what you want.

On the simplest level, then, the choice comes down to pushing your goalkeeper forward to create space out wide or keeping him deep to create space between the lines. As passing goalkeepers continue to get more popular, it’s still an open question as to which kind of risks and rewards they want their teams to create.

Premier League goalkeepers keep passing straight to the opposition – what’s going on?

• The Athletic report