By having a back court setter moving into the space between positions 2 and 3, you open up the reverse / backwards set to an attacker at position 2 (see picture to the right). The benefit in doing this is that it means that you have 3 team mates who can attack in the front zone of the court!
Shown above is a team who are using a team system with a setter penetrating into the front zone, from the back zone. This is usually done with a player from position 1 or position 6. To do this you must have a setter who is comfortable and quick at moving around the court.
By having 3 attackers in the front zone of the court it means that you can spread the attack right across the entire length of the net making it much harder for the blockers.
This is a complicated concept and should only be used when the players are ready.
Also, make sure you communicate so you don't have 2 setters!