A defender facing an attacker one against one is rarely trying to win the ball immediately. He is trying to remove options until only the least dangerous one remains.
The stance decides the direction
By angling his body, a defender closes one side of the attacker and leaves the other open. The attacker takes the open side because it is the only one available without a trick.
This makes the duel predictable, and predictability is what allows a defender to time a tackle rather than react to one.
The angle is set as the defender approaches, not once the attacker has begun to move, which is why the first two steps of a defensive approach matter more than the tackle.
Weaker foot narrows the finishing options
An attacker forced onto his weaker side loses shooting power, crossing accuracy and the ability to disguise the delivery.
He also tends to need an extra touch to reposition the ball, and that touch is the moment a covering defender arrives or a goalkeeper narrows the angle.
The reduction in threat is often larger than any tackle would achieve, and it carries none of the risk of committing.
Direction is a team decision, not an individual one
Showing an attacker outside sends him towards the touchline and away from goal, but it concedes crosses. Showing him inside removes the cross but leads him towards the centre.
Which is correct depends on what waits behind: a team with dominant centre-backs may happily concede crosses, while a team with a congested midfield prefers to funnel play inside.
The important thing is that everyone in the defensive unit applies the same rule, because a defender showing inside next to one showing outside creates a gap between them.
Distance controls how much the attacker can do
Approaching too close allows the attacker to knock the ball past and use his body, since the defender has no room to turn.
Standing too far off gives him time to pick a pass or shoot without pressure, which defeats the purpose of the engagement.
The workable distance is roughly one long stride, close enough to influence the touch and far enough to react to a change of direction.
Speed of approach matters more than speed of tackle
A defender who sprints all the way to the attacker cannot stop or change direction, and a simple touch past him ends the duel.
The coached pattern is to close the distance quickly and then decelerate over the final metres, arriving under control with the feet active.
Defenders who do this well rarely appear in highlight reels, because the duels they win end without a tackle and without a photograph.