Defending as a Unit Explained - Invisible String
The first job of our players when out of possession is to stop the ball getting into dangerous areas where our opponents can potentially create goalscoring chances.
The second job is then to win the ball when the opportunity arises.
We're more likely to be able to do the second thing if our opponents make a mistake and we increase the chances of that mistake happening if we frustrate our opposition into trying to force something that isn't on or force them into an error by putting them under pressure. If we block off any obvious forward passes into dangerous areas, the opponent is more likely to eventually try something risky.
So we need to stay compact and not leave any obvious gaps for the opposition to play dangerous forward passes into.
Describing to the players that they're linked by an invisible piece of string that pulls them across the pitch as a unit, can be a good visual way of explaining what's expected. The angles and distances can change, but the string continues to link the players and if one player goes too far out of position, the metaphorical string will snap. The diagrams below show what we mean by this.
Image 1: the opposition have the ball in a central area, so our players stay central, protecting the goal and blocking any dangerous forward ball in to the striker, as well as being able to shift across quickly if the ball is played wide. Invisible string shown by the yellow dotted line - note positioning of players mean a dangerous ball through them or a shot on goal would be very difficult from here.
Image 2: the ball has been played to the left of the pitch, so our unit move across to cover. Distances between players stay approximately the same but the angles change to provide maximum cover and protection. Players are still blocking the goal, a ball into the striker isn't really on. The opposition either have to go backwards or sideways, or force something that is likely to lead to losing the ball.
Image 3: the ball has been switched by the whites in an attempt to find an opening. As the ball is travelling, our defending team move across as a unit and once again, distances and angles between players are still such that the team are compact, hard to break down, blocking the goal and the danger area in front of goal. Invisible string still there in yellow.