It happens that the opponent has a very dangerous backcourt player, who makes goals as he pleases AND makes his teammates free. In order to take out such a player from the game a defender is placed close to that player, so a pass to him is dangerous. The defending player, who marks the attacker must be quick on the feet in order to position himselves in front of the attacker all the time and have great feeling of the game behind. This makes it possible to intervene and catch the ball during a pass.
Don't look the marked player in the eyes, but downwards and keep an eye on the body movements and movement of his feet. They often indicates which way the marked player indents to move.

Golden rule: The attacker must never get behind the defender.

When you mark you must not touch or check the opponent, but only place yourselves in such a way, that you stop his movement against goal. But should the marked player receive the ball, you are of course allowed to check.

If the dangerous player is moved to the wing position or as a circle runner, he must be treated as an ordinary player and a personal marking is lifted. If you follow the attacker as a marking player it can be used by the attackers to create a shooting solution where you should have been. If the player is moved to the wing position you can chose to cover high, but if the wing runs in without a ball, he must be treated as an ordinary player. The same goes if he is a circle runner.