The idea with 3-3 defense is to put a pressure on the attackers and try to conquer the ball.
If the attackers are unable to run into a free position, the player with the ball is in an unfortunately situation and is forced to make a dangerous pass.
3-3 is a very aggressive type of defense, which demands fast players, who are always able to be between the attacker and the goal.
If the attacker comes behind the defender - with or without - the ball it wil be rather easy to score or get into a position to score, if the attacker receives the ball.