When a team is in a superior number situation it often happens that the opponents scores even if they are less one player.
The reason could be that the defense takes it too easy because they think that 6 defenders against 5 attackers are in their favour and that there is no reason to be alert.
Opposite that the opponents know that they must really do something in order to score or defend themselves.
It is therefore a good idea to make it as difficult as possible for the outnumbered attackers and try to provoke passive play or conquer the ball.

In this example the players choose man-to-man defense and the remaining defender takes care of the center area.
The wings are covered for passing and by this the remaining defender can concentrate to be backup in the center defense.
The main rule in this defense is that no attackers must come behind their defender.