Something very odd happened at the end of last week. Historical changes in tactics usually happen incrementally over time: there would be rumours of a side doing something unusual ("and they say the centre-forward plays in midfield …") followed by a pivotal game in which that tactical change proves decisive and is accepted as a new phase in football's development (Nandor Hidegkuti picks England apart while scoring three in Hungary's 6-3 win at Wembley in 1953).

More recently, perhaps, with the blanket televisation of football, it's been possible to trace the evolution, but still, there tends to be one moment, one game in which everything snaps into focus (Lionel Messi, for instance, ripping Real Madrid apart from the false nine position).

Last Thursday night, we saw something highly unusual: a 3-1-4-2 from Universidad de Chile as they won the first leg of the Copa Sudamericana away to Liga de Quito (they're likely to revert to orthodox 3-4-3 for Wednesday's second leg). We'd seen 3-3-2-2 and 3-3-1-3 before – usually from Marcelo Bielsa sides – but this, with the wing-backs pushed so high up they were midfielders and the deep-lying midfielder such a holder he was effectively an auxiliary centre-back, was new.

Three-and-a-Half Men and the Rwanda Protocol

But then Tom Legg, whom I'm going to go out on a limb and describe as east Africa's leading tactical analyst, got in touch to say that earlier the same Thursday, Rwanda had switched to a 3-1-4-2 in the second half of their Cecafa Cup semi-final against Sudan. Rwanda had contained a narrow Sudan relatively comfortably in the first half to lead 1-0, but Sudan's half-time switch from a 4-4-2 diamond to 4-2-3-1 gave them more attacking width and led to them equalise after 68 minutes.

Rwanda's Serbian coach, Milutin Sredojevic, promptly withdrew the 17-year-old centre-back Emery Bayisenge and replaced him with a 17-year-old midfielder, Andrew Buteera. Buteera is usually thought of as a creator, but here was deployed deep as Rwanda shifted to a 3-1-4-2. As Legg points out, this was counterintuitive: if your opponent is winning the battle on the flanks, making your own team even more narrow seems like the last thing you should do. As it turned out, though, Rwanda retook control of the centre, cutting the supply to Sudan's wide men. Buteera and the two wide men, Jean-Claude Iranzi and Eric Gasana, found space to create attacks and Olivier Karekezi lashed in the winner from a narrow angle with 13 minutes remaining.

That was surprising enough – an unexpected formation popping up on the same day in Ecuador and Tanzania. But the biggest surprise came on Saturday as it, in slightly mutated form, appeared again in Spain, and specifically in Madrid, not in a regional African tournament or South America's secondary competition, but in the biggest game of them all: El Clásico. It was as if M Night Shyamalan were directing a documentary on football tactics, the 3-1-4-2 virus sprouting uncontrollably across the globe.

As has now been well-documented, El Clásico turned on Pep Guardiola's decision midway through the first half to abandon the slightly odd 4-3-Cesc Fábregas-2 with which he had begun. He pushed Dani Alves from right-back to right midfield, so he could check the runs of Marcelo and cut off the support for Cristiano Ronaldo, a move that also spared him from having to pretend he is a full-back which, as anybody who has seen him play for Brazil will know, he is not.

That meant Carles Puyol moving to right-back, with Sergio Busquets dropping in to become a second centre-back. Gerard Piqué became the right-sided centre-back, allowing him to double up on Ronaldo when required, while Alexis Sánchez moved to become a highly mobile centre-forward (a false nine, if you like, but with lateral rather than longitudinal movement). Messi operated as an orthodox 10, with Andrés Iniesta shuttling on the left and Fábregas brought back much closer to Xavi Hernández. At first, the formation looked like a highly fluent 4-2-3-1, as though Barça were going to match Real Madrid shape for shape, but then the real benefit of the system became clear.

It is often overlooked just how key Busquets is to initiating Barça attacks, but he is always there as the get-out: if a player gets into trouble, he can go back to Busquets. Block off the escape route, though, and anxiety can be induced. Attack the metronome and the whole orchestra loses rhythm. José Mourinho surprised many by opting for a 4-2-3-1 rather than a 4-3-3, but what it allowed him to do was press with five men, leaving Lassana Diarra to protect the back four. That brought the opening goal, but it also rattled Barça.

Moving Busquets back, though, gave him time and space. Withdrawn from the front line, he could begin again to shape the battle. It was a risk, because it left Mesut Ozil free, but he is a slightly old-fashioned playmaker, somebody who is adept at finding time amid the hubbub to measure a pass. Usually that is an asset, but here it gave Busquets time to close him down. We are used to seeing Busquets dropping back from midfield to become a centre-back; but here he was doing the opposite, stepping out from the back four to become a midfielder. Perhaps this is the logical outcome for a side that flips so often between a back three and a back four: it ends up playing a back three-and-a-half.

First principles

But there is a deeper logic, and one that could be predicted. When Jack Charlton made his famous comment after the 1994 World Cup about full-backs being the most important position on the field, he was specifically referring to the fact that when two 4-4-2s clash, the full-backs are the players with a direct opponent. They are special not because of anything inherent in being a full-back, but because they are the players with the time and space to shape the game.

Football has moved on, though, and the prevalence of 4-2-3-1 and 4-3-3 means that full-backs often do have a direct opponent. That can create fascinating tactical duels – Roberto Carlos v David Beckham as Real Madrid beat Manchester United 3-1 in 2003, Theo Walcott v Danijel Pranjic as England won 4-1 in Croatia in 2008, Michael Essien v Cristiano Ronaldo in the Champions League final in 2008, Gareth Bale v Maicon as Tottenham beat Internazionale last season – but it also means that the space that was once the full-back's birthright is no longer guaranteed.

So where is that space? If a team plays a back four against an opponent with a lone central striker (or a false nine), then at least one of its centre-backs should be spare. It's not quite the same as a full-back being free in that it's clearly far easier for a single striker to shuffle 10 yards to close down the other centre-back than it is for him to run 30 yards to close down a full-back, but two centre-backs faced with one forward trying to close them down should be able to work space for one of them to step forward with the ball, at least until a potential presser from the opposing midfield comes into play.

Two years ago, I suggested we would increasingly see Piqué start to step forward with the ball to join Busquets in midfield; actually the reverse has happened and we have seen Busquets drop back to join Piqué (the clue was in the influence of Bielsa on Guardiola; the Argentinian visionary, also an inspiration for Universidad de Chile's coach Jorge Sampaoli, has a habit of pulling midfielders back into defence, as he has done with Javi Martínez at Athletic Bilbao). The effect is the same, a central defender who steps out from the back, a playmaker from the centre of defence.

Of course that is not entirely new. You could go back to Martim Francisco, the coach of Vila Nova, a club from Nova Lima, a town about 20 miles from Belo Horizonte, in the early 1950s. He pushed his left-half, Lito, back to play as the quarto zagueiro – the fourth defender – a term still used in Brazil for a centre-back with a responsibility to step up into the midfield. More obviously, there is Franz Beckenbauer and a whole generation of liberi stretching through the 70s and 80s all the way to the likes of Miodrag Belodedici and Matthias Sammer in the 90s. Sammer, though, was very much the last of his kind, and the libero has not really existed for 15-20 years, squeezed out by the influence of Arrigo Sacchi and the love of the hard press.

Germany, generally, was slow to respond to the rise of high pressing with a back four, which was why Volker Finke had such success with Freiburg. Christoph Biermann argued in Der Ball ist Rund that, for all Berti Vogts's faults, he did at least reconcile the Germany national team to the modern world of pressing (Borussia Dortmund, of course, pressed ferociously last season, yet Bayern were clearly unsettled by Augsburg's high line in their defeat there two weeks ago).

An environment of change

And that is what makes Busquets's role so fascinating – it facilitates a back three-and-a-half in a system that presses. Again, there is a precedent, and perhaps it is not surprising that it should be found in the heritage of this Barça: Johan Cruyff's Barcelona Dream Team of the early 90s, although they also lined up in a 4-3-3 or a 4-4-2, often played a 3-1-3-3, as did the Holland of Guus Hiddink. In Cruyff's variant, Ronaldo Koeman was often the one, although Guardiola himself operated there on occasions; but at Euro 96 Hiddink was playing Clarence Seedorf in the role.

So Guardiola, to an extent, has gone back to his roots, although there is a difference between the roles of Koeman and Busquets, if only in how opponents line up against them. Where Cruyff's 3-1-3-3 was rooted in the Rinus Michels belief that you played as many defenders as the opponent had forwards, plus one, and so Koeman was effectively free until there was an attack down the flank at which he had to drop back to become a second centre-back, Guardiola's – on Saturday at least – was predicated on Busquets staying deep (like Koeman deeper than the opposition midfield, but actually deeper than Koeman because the general trend from three-band to four-band systems means the first wave of opposing midfield tends to play higher) and stepping up when the opposing playmaker came into the game.

And that brings us to the other recent sighting of the back three-and-a-half, which was in the Spanish Super Cup. Applying first principles to the issue of tackling a false nine, it makes sense to play a back three and track the opposing false nine with the player who would have been the second centre-back, whether a defender or a holding midfielder. That was precisely how Mourinho deployed Ricardo Carvalho against Messi, with some success, even if the result ended up going against Real Madrid – again, the result being a back three with a player who often played in the back line but did not remain there.

So that gives us four examples this season, of teams playing a 3-1-4-2. All had different motivations. For Real Madrid, it was to free a player to man-mark. For Universidad de Chile, it was to provide the cover that allowed the wing-backs to engage Liga de Quito's wing-backs high up the pitch. For Rwanda it was to wrest control of the midfield through weight of numbers in the centre. For Barcelona it was to create space for their conductor. If there are four separate routes to a single solution, that suggests there is not a sole cause.

So, why now? It comes back, as tactics always do, to space. The prevalence of systems with one or no central strikers means that for much of the last decade, one of the centre-backs has been spare. To an extent, that's quite useful in itself, providing additional defensive cover. But there are more interesting things that can be done with him, and it is that that football is only just beginning to explore.