The point of a high line is to press heavily, close down the opposition and ideally win the ball back in an advanced position, therefore launching a swift counter attack.
This worked perfectly for André Villas-Boas at Porto; however it has proved troublesome at Chelsea.
Let's start from the beginning of City's goal-scoring sequence.
David Silva makes a routine pass to Pablo Zabaleta.
Juan Mata isn't marking Zabaleta, even though the Spaniard is out wide and Chelsea aren't in possession; Zabaleta passes it to Sergio Agüero.
John Terry, thinking he can win the ball, rushes out to pressure Agüero, and Ashley Cole is there too.
Note this, he's on the right side of Agüero, and one has to ask why isn't he on the left side?
Oriol Romeu pushes forward.
Mata's decision not to mark Zabaleta has caused a chain of events where Agüero has drawn three Chelsea players out of position.
So what happens next?