Just as an Offensive coach changes around his formation to strengthen some plays, you can select different defensive formations to counter his play.
Most often, unless you just know he’s going to run (call Short Yardage or Goal Line) or pass (call Nickel), most teams stick with the 3-4 or 4-3, based on their linebackers.
As you change your formation, the play diagram updates to reflect your choice.
The 4-3 (4-3-4) can be a little better against inside runs and has a good pass rush, but one less linebacker who could be in pass coverage. MLB is the key linebacker. Line can shift.
The 3-4 (3-4-4) requires that you have four good linebackers, with the ROLB being key. Better against short passes and sweeps, although it does allow more runs through the line.
Short Yardage is extremely tough against runs, but susceptible to play action passes. In Goal Line, the men are even closer to the line, so use it only near the goal line. Can shift your line.
The Nickel defense replaces a linebacker with another defensive back, to improve pass coverage. Can still react to runs, but weak against draws and outside runs.
The Prevent defense has only three men rushing, and everyone dropping back to prevent long gains. It’s open to short and medium passes.