In the football match, the defender's player runs in the opposite direction in the penalty area tacitly, creating the "offside" of the attacker. Is this a foul?

this is not a foul, this is an offside. Offside Trap refers to a defensive tactic in football match. According to the rules, at the moment when the attacker passes the ball to his own player, if the player who is going to catch the ball is in any position between the last defender of the opponent and the goalkeeper of the opponent, it is offside. Using this rule, the defender suddenly sprints to the opponent's half-court at the moment when the opponent passes the ball, resulting in the situation that there is no defender (except the goalkeeper) between the opponent's receiver and his own end line, which makes the opponent offside foul. However, it is difficult to implement this tactic, because everyone must be of the same mind. If one of their own players fails to keep up with the defense line during the execution, the whole defense line will be useless, so that the other side can take advantage of it.