The defense system has that purpose to make a half court pressure on all the attackers and to direct the ball to the wings followed by a stealing of the ball.
When the opponents give up the ball, the defending players place themselves as close as possible to the attackers (at least 3 meters) and when the attack starts, all are covered, which creates good possibilities for stealing the ball.
But remember - it is also easily for the attackers to get behind the defense and then there is a free way to the goal.

The defense can also be started, when the attackers are on their own half of the court and the defense is placed just outside the free-throw line and outward.

The defending players must be very agile, both sideways and forward and backward.
They must also be very good in tackling, be able to foresee the attackers moves and of course - no attacker must come behind the defending player.

In general the CB is covered very closely in order to prevent her to receive the ball from backcourts and wings, and the backcourts are covered slantwise so that the only possible way of breaking through is on the outside of the defense.