All defenders should stay on the middle line of the field except one who will serve as a back up in case the ball comes back.
Thank youCorrectionAll the defenders should push up to be at or near midfield. No defender except the goalkeeper should be back. Remember that the defenders will, in general, form a fairly straight line (across the field) because anything else may leave a defender behind his own line of defensive team-mates. And that defender, because he is behind his team-mates, is the new "second-to-last defender" and will be the offside mark. This allows an attacking player to get behind a defender in another portion of the field and still be onside. Any ball played up ("kicked up") in the area where the attacker has slipped past the defender (but is still onside because of the other defender is "playing back to be a back up") will almost certainly be swooped on by that attacker and a clear advantage will appear. That forward is off to the races because the defender near him is now behind him. He's going to the ball and has nothing in front of him but the keeper and the goal. The defender who has stayed back has created an opportunity for the other team. Not a smart thing to do. Defenders generally move up and back as a unit to create the offside line.
Spend a few minutes watching a top flight soccer match and evaluating the location and movement of the backs. The defenders form a fairly straight line across the field and move up and back pretty much in that line. They know instinctively that the furthest back of their number is the second-to-the-last defender, and no single one of them will permit himself to "play back to be a back up" because it sets up the dangerous condition described above. Anyone who had properly coached defenders or has played in the back on a competitive team knows it can be a fatal error to have a defender "playing back to be a back up."
And you're welcome.