Defensive positioning is often reduced to staying goal side, which is only half of the instruction. The other half decides whether a defender ever wins the ball rather than merely following his man.
Goal side is the safety requirement
A defender positioned between his opponent and the goal cannot be beaten by a simple pass in behind, because he is already occupying the space that pass would use.
This is the default for defenders near their own goal, where the cost of being wrong is immediate and the priority is preventing the worst outcome.
Purely goal-side marking is reactive, though. The defender is always following, and he only touches the ball once his opponent already has it.
Ball side is the aggressive requirement
Shifting slightly towards the ball puts the defender in the passing lane rather than behind it, which allows an interception instead of a tackle.
Interceptions are far more valuable than tackles because the ball is won while the defender is facing forward and the opponent is out of position.
The exposure is that a well-timed movement behind the defender gets past him, since he has traded some of his covering position for proximity to the ball.
The correct balance shifts with distance from goal
High up the pitch, a defender can afford to be ball side because the space behind him is covered by distance and by the goalkeeper's starting position.
Inside the penalty area the calculation reverses, and defenders sit firmly goal side because an interception attempt that fails leaves a shooting opportunity.
Watching a defender's position change as play moves up the pitch is therefore a reliable indicator of how well he understands risk.
Body orientation resolves the conflict
A defender who stands side-on can see both the ball and his opponent at the same time, which allows him to hold a ball-side position without losing track of the runner.
Standing square to the ball means checking over the shoulder repeatedly, and the runner moves during every check.
This is why coaches spend so much time on the open stance, since it is the mechanism that makes the aggressive position survivable.
Cover changes what an individual can attempt
A defender with a covering teammate behind him can afford a ball-side position, because a failed interception meets another defender rather than open space.
Without cover, the same position is reckless. The individual decision is therefore determined by the team's structure rather than by the defender's preference.
Defences that appear to press aggressively from the front are usually organised so that every aggressive marker has a covering player behind him.