In the heady days of Rafael van der Vaart, Luka Modric, and Gareth Bale wide, expansive attacking was the order of the day at White Hart Lane. All of Spurs' key players thrived on having plenty of space in which to work whether it was Bale and Aaron Lennon needing space to run, van der Vaart needing space to receive the ball, or Modric needing the team spread out to maximize his passing ability. When those Tottenham teams became narrow, they struggled. At their best, they spread the field and became almost impossible to contain due to their combination of clever passing and frenetic movement.
Thus one of the most interesting transformations of the team as it has been built by Mauricio Pochettino is the team's embrace of an incredibly narrow 4-2-3-1 shape. Below you can see both the old and new shapes set next to each other:
The reason for the shift is simple: The Redknapp Spurs wanted to play fast from deep positions and on the counter. So ideally you'd have everyone save van der Vaart and Adebayor behind the halfway line until the ball is won. Typically Parker would win the ball, get it to Modric quickly, and then Modric would look for Bale and Lennon storming down the flanks and the break would be on.
The Pochettino Spurs, in contrast, want to win the ball in the attacking half of the field and transition it to attack quickly. But in order to win the ball in that part of the field they need to be able to pressure the ball with numbers. If only one player is chasing the ball and no one else is nearby, then the opponent can pass their way out of trouble quickly. You need quick, immediate pressure from multiple players for Pochettino's system to work. This, in turn, necessitates a narrower shape for the team so that they can, essentially, hunt in packs.
In practice you can see how this looks in the screen capture below which is taken moments before Spurs scored their second goal in Monday's win against Aston Villa. The blue arrow indicates where Christian Eriksen is going to go in order to win the ball.:
As you can see above, midfielder Deli Alli has actually pushed into a more advanced position ahead of the three-man band of attacking midfielders. Lamela, Dembele, and Eriksen are all within about 10-15 yards of each other plus Alli is also in the same general vicinity.
There are times when this narrowness can be a liability. If you pack four players within 15 yards of each other it is easier to mark all four of them. However, the flip side to this is that when the ball is in-between possession it is much easier for Spurs to converge on the ball with multiple players and force a turnover. Once the ball is back in Spurs possession, they can create a scoring chance if they move quickly enough.
The gif below shows how this goal happened and, significantly, how quickly it happens from the time Spurs win possession:
From the time the ball is won till it ends up in the net, Spurs take seven touches in seven seconds. Eriksen takes two before playing a pass to Rose. Rose simply crosses the ball first time, Kane wins the header, Alli takes one touch to control, and then shoots. The speed of the attack means that Villa never gets set defensively, Rose has acres of space to play a cross, and Alli has just enough time on the ball to take his shot. (To be sure, full marks to Alli for scoring this one – it's not a great scoring chance given the number of people between him and the goal and where he is taking the shot from.)
This goal is thus a simple illustration of how Tottenham's narrow shape complemented by attacking fullbacks makes Pochettino's system go. The ball is turned over in the advanced central area but is won by Spurs' left sided attacker thanks to his central, tucked-in position. Villa is also a little sloppy immediately after winning possession due in no small part to Alli and Dembele's proximity to the ball.
To be sure, narrow play can stagnate, particularly if the fullbacks do not get forward. But in this case we're seeing how the system is meant to work. Having multiple players in close proximity to one another makes it easy to win the ball back when it is lost. And when you can combine that ability to win the ball back in advanced positions with the sort of quick, clinical attacking on display above you have a fairly potent combination.