For the first 25 minutes or so against Swansea City, it was obvious that something was going horrendously wrong with Chelsea's game plan. And it was, more or less, equally obvious what that was: our pressing game was completely incoherent.
Before we dive into the analysis, some evidence that I'm not completely mad might be in order. So here's Jose Mourinho himself, discussing what went wrong and what went right in the post-game.
The first half was hard. The players couldn’t find the references to press so Swansea had too much time and too much space which makes it easy to play football. In the second half the way we pressed them completely changed the game. We had the ball and with the intensity we played in the second half it was difficult for them.
'Pressing' in football is essentially an attempt to win the ball or force a mistake high up the pitch with non-defensive players. It's the opposite of standing off the opposition and letting them come forward, and tends to be used in conjunction with a high defensive line (for reasons that will become apparent a little later). Pressing football is associated with fast attacking play -- forcing a turnover high up the pitch tends to lead quickly to goalscoring chances, but it's not always that simple, and sadly we don't have time to really dig into the details.
There are two aspects worth diving into, however. The first is the cohesion of the team implementing a press. The risks involved are obvious -- if you apply pressure on the ball with attacking players but don't push the defenders up into the vacant space, short passes open up in midfield for the team in possession to hit. Once that pass is played, the midfield shield has been bypassed, the supporting pressers are all moving in the wrong direction* and the defence is square against a team that has room to run into. This is very bad.
*Football is essentially a game of space and inertia, with players' inertia dictating where space will open up in the immediate future. Watch the game with this in mind and you're most of the way there.
If your defenders decide it's time to press and the attackers don't follow suit, which is rarer, it can be even more of a disaster. All but the least technically capable teams in the Premier League will immediately send one of their forwards on a diagonal run cutting through a channel and scoop a ball over the top to try to hit them. Failure to put pressure on the ball while the defence is stepping up is suicidal.
Since the energy used and risk involved in playing a press is higher than not, teams can't do it all the time. The need for a unified team press rather than an individual one motivates the use of opposition-specific pressing triggers (or references, if you're Mourinho). Barcelona, long undisputed masters of the high press, used several -- a miscontrolled pass, a change of possession, a backwards pass -- basically any situation in which a team is uncomfortable with the ball is a good time to hit them.
And although that sounds simple, in reality it can be quite complex. Knowing who to press, when and how hard is the base of the team's whole defensive strategy, and if that's not understood, bad things happen. For instance, we get something like this:
Here Cesc Fabregas races up from deep midfield to press Jonjo Shelvey ... who, since nobody else in a Blue shirt is putting any pressure on the ball, can simply tap back to Ashley Williams, who's already preparing to hit the space in midfield that Fabregas has vacated. The front five is immediately bypassed by a pass that even a centre half could hit in their sleep, and within ten seconds Bafetimbi Gomis has a half-chance at the other end of the pitch.
Chelsea were actually fine in the first half when everyone was on the same page, but this incoherence happened far too often. Here, for instance, is Nemanja Matic getting drawn up the pitch chasing the ball, leaving the defence completely unshielded and passing lanes through the centre open:
Yes, that is Matic alongside Diego Costa, which is definitely not where Matices should live. I wish they'd had a camera on Mourinho's face while this was going on, because I imagine we'd have had some fun with lip-reading there.
Note that the above stills are team failings rather than a individual ones. In isolation, Fabregas pressing Shelvey and Matic chasing the ball back are not mistakes (ok, ok, Matic chasing the ball back that far is probably indefensible, but that's not the point). They only end up hurting the team when everyone's on different pages. This is why Mourinho hauled both Eden Hazard and Fabregas to one side in the middle of the first half to explain what he needed; this is why he spent halftime explaining to his team exactly what pressing references he was hoping to see them use.
After the break (Ramires had come on for the disappointing André Schürrle), Chelsea looked like a completely different side. With Swansea under genuine pressure, they couldn't control the ball in their own half, and with the defence and attack acting as a unit rather than two wildly-flailing arms, there was no easily-exploitable space for the visitors to hit. If the first half was a disaster, the second was virtually perfect.