The role of a full-back is constantly changing. Earlier they only had defensive duties, later they became an essential part of the offensive movements. Sometimes they make overlap runs, sometimes they only make underlap runs, mostly they give width, although nowadays we often see them in the halfspaces, in a false full-back role. Therefore a modern full-back must be versatile in order to be effective in every different system (for example David Alaba, Philipp Lahm, Daniel Alves).
In my opinion, the most important aspect of this tactical novelty is that it gives flexibility to the playing system. Full-backs may be deeper, may be higher in the halfspaces, they may occupy the flanks to provide wide options in order to stretch the opponent horizontally. It is recommended to pay attention to Alaba's movements in every game. Most of the time he occupies the left halfspace deeper, to maintain a great connection with the left-winger, although when the CM drops deeper, he moves higher between the lines in the halfspace, to provide attacking depth support etc. (remember that he scored against Arsenal from the centre, confirming his versatility).
Basically, that's how the positional structure looks like against a horizontally compact 4-4-2 (Wolfsburg-Bayern games). The FBs are narrower, which means:
- more horizontal circulations at the back (to open up vertical passing lanes)
- better connections with both the CBs and the Ws
- increases the possibility of diagonality
- makes transition play easier, as FBs are already in position (defensive line may be narrower instantly, to cut the opportunity of through balls/buy time for the team to reorganize)
- easier 1v1 situations for the Ws
A strategical advantage is that the FBs positioning in the halfspaces makes possible to circulate constantly between the halfspaces (stretching the opponent's defensive shape horizontally). With this the offensive shape is a 2-3-5, which have 2 different forms. One, where the 3 players are the LCM-DM-RCM (as CMs are basically better to dictate the tempo, and circulate), and one, where the 3 are the LB-DM-RB (mostly at Bayern Munich).
Here are some examples from Bayern games:
An important aspect of the offensive shape is to be able to counter-press in case of losing the ball. As the FBs are occupying the halfspaces, they are able to close down the most important channel, to prevent possible counter-attacks (as most counters go through the halfspaces or the centre). If the counter-press does not succeed, then they are easily able to move back and give stability in the defensive-line.
Flexible FB movements:
When the CM drops, the FB must move higher (to provide attacking depth/vertical or diagonal option), and he has 2 options:
- as the CM drops, the halfspace between the lines is open, the W moves into the halfspace, the FB moves to give width down the flanks
- the FB moves between the lines (in the halfspace), the W gives width -> from here the LB has the option to make underlap runs as soon as the W gets the ball (usual combination between Thiago-Alaba-Coman)
I think this is a very intriguing aspect of the game, as full-backs became an essential part of an effective positional play through wide combinations.