The following screenshots should make it clear to anyone of what the DL actually does. Also how CD and mentality works together with it.
We start of by showing a picture of the pitch. This shows how the yellow team, playing from right to left could set their DL. The black lines shows approximitly where on the pitch the different numbers places the DL.
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The second picture shows the team starts to defend to try to win the ball back, at the same moment the oppositon enters the team`s own half. Notice this is the second half and the teams have changed sides.
Player No.25 and 9 has individual mentality set to 12. Two notches below the set DL. This makes them position a little deeper than the half-line. The whole team is taking up defensive positions according to the DL.
Still their closing down is set to higher than the DL. This ensures that they will close down. They also have two notches higher CD than the No.8(DMC). Combined with one notch higher mentality. In the build up to the screenshot. The No.25 and 9 were wider, and if the DMC had been given higher mentality and closing down. He would might go in for the tackle. But as soon as the player with the ball entered their half, they closed in to him. This makes the DMC stays behind them and act more as a safety if the player with the ball should dribble past No.25 and 9. Also this way the DMC is able to secure the space behind the two MC`s and the defenders.
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What is important to understand is that the DL(the black line) set will make every player on your team start to defend from it. The next screenshot from another match will explain further.
Here we see the whole team has moved into the opposition half during an attack. They regain the ball and quickly start an attack. With a DL set to 14 we see that the defenders action is to stay close to the DL and defend from there. Whilst a deeper DL setting would suggest that they immeaditely should move back to defend from that area.
This is a major guideline for all your players on the pitch. The players will follow this instruction stricter than individual mentality and closing down.
If you don`t get the players positioning(with formation) and individual mentality/closing down set right in adjactence with DL you will face tactical problems.
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I would say first set your DL. Then set your individual mentality and closing dwon instructions accordingly. This way you can shape your players positioning and formation. Also it gives you the ability to choose wich players will close down in what order.