This isn't the full story of the team's current tactics, but rather an introduction to the critical problem that formations and strategy tries to address - the correct usage and weighting of both occupied and unoccupied space.
4-4-2 is the standard shape for English teams, and has been for some time. For those of you unfamiliar with tactical terminology, the nomenclature for team shape goes something like number of defenders-number of midfielders-number of forwards, with more bands being added as appropriate. Obviously, just saying '4-4-2' misses a lot of subtleties in terms of the shape of that team, and there's an infinite variety of playing styles a formation can use. Here I present 4-4-2 in its typical usage: Two central defenders, two wide defenders, two central midfielders, two wide midfielders, and two forwards. The goalkeeper is not named.
Anyway, about half of the teams in the Premier League favour the 4-4-2. They do so because it's versatile, strong in defence, and uses space extremely effectively, spreading the team across the pitch while maintaining a sturdy pedestal from which to launch attacks. Teams will typically use a defensive midfielder coupled with a more adventurous box to box type as their central pairing, and they'll frequently experiment with the positioning of strikers whilst still maintaining the 4-4-2. Why? Well, because it's an effective formation, most of the time. It's especially effective against other 4-4-2s, and can adapt to deal with many other formations by simple tweaks to the system.
But while the 4-4-2 is useful at spreading across space, it values all of that space equally. This is clearly wrong - as anyone who's every watched a match can tell you, there are certain areas of the field which require more care and attention than others. And here lies the main, crippling weakness of the system - the 4-4-2's weak heart is the two-man midfield core.
If we are to assume that other players remain in their rigid slots (otherwise gaps are created elsewhere, which are exploitable), there are only two players in the very centre of the field in a 4-4-2 system. Two is all very well and good against two, but there are now multiple formations which include three central midfielders - the 4-2-3-1 is a favourite right now, the second band containing two central defensive players and the third a central attacker. What happens when two play against three? Well, the three have the ball a lot. It's almost impossible for two players to regain the ball after losing it against a passing triangle - one of the corners can be cut out and the ball can be pressured, but that's all.
Figure 1: Two opposing central midfielders trying to mark the Chelsea triangle.
As a result, keeping possession is much easier for the 4-2-3-1, or indeed Chelsea's current 4-3-3 system. By outnumbering a team in a critical area, the Blues are getting a major advantage, to the point where they can happily accept a hole somewhere else on the field. But the hole isn't actually too problematic - you can also use the absence of a player to great effect in the patient build-up play a team can put into practice once they've won the midfield war.
I've mentioned that I consider the central midfield to be the most important area of the pitch, but that changes dramatically once the team has secured control of the ball. Suddenly, the forward areas open up. With only one striker and two wide players as compared to the two strikers and two wides, the non 4-4-2 system would seem to be at a disadvantage - a simple numbers game, just like in the midfield. However, by exploiting the defenders tendency to guard zones rather than players, clever runs from a forward three can actually disrupt the defensive shape and leave forward zones open for supporting players. With four attackers, defences know what to do. Three dynamic, interchanging forwards are much harder to track, and since the ball-suppliers have time on the ball, they can wait for the right moment to pick out their forward teammates.
It's probably 4-4-2 that's responsible for the long-ball game played by many. While the long aerial pass isn't exactly unknown to anyone, some teams play it much more often than others - and those are generally the teams who play in a 4-4-2 system. Why? Because it bypasses the centre, where passes can be bogged down by roving gangs of opposing players, and favours a direct route to the strikers, who are now on even terms again. By its very nature, 4-4-2 avoids the centre, and the system generates quick wingers, hard-working box-to-box midfielders, and tall, tough strikers. Playing 4-4-2, in other words, leads to the development of physical players rather than technical ones.
Physical play can win the day, of course. It's the most natural shape for many teams, but managers must adapt their shape for their personnel. If you try to replace a box-to-box midfielder with an attacking playmaker in the midfield core without a corresponding change in team shape, your team will be overrun. If you forced Chelsea back into the 4-4-2 box, Michael Essien and Frank Lampard (non-injured edition) would suddenly look a lot less effective in the centre - if they were even still playing.
Team shape is an expression of the players you have, the style you wish to play, and a reaction to the opposition. Chelsea are packed to the gills with skillful, intelligent players, and the team is designed to maximise the advantage they already have over their opponents by choosing the correct team shape. Often, the other manager will change his side in response - Stoke's 3-2-3-2 against the Blues at Stamford Bridge is a great example - but as the favourites Chelsea typically get to dictate play. And as Jose Mourinho has said (and at the risk of repeating Zonal Marking):
‘Look, if I have a triangle in midfield – Claude Makelele behind and two others just in front – I will always have an advantage against a pure 4-4-2 where the central midfielders are side by side. That’s because I will always have an extra man. It starts with Makelele, who is between the lines. If nobody comes to him he can see the whole pitch and has time. If he gets closed down it means one of the two other central midfielders is open. If they are closed down and the other team’s wingers come inside to help, it means there is space now for us on the flank, either for our own wingers or for our full-backs. There is nothing a pure 4-4-2 can do to stop things’
Right now, Chelsea are playing a 4-3-3, with the second band exclusively consisting of a midfield triangle:Jon Obi Mikel, Michael Essien, and Frank Lampard. Go ahead and try to stop that with two central midfielders.