As a setter, you are not always limited to setting to teammates in front of you. You can send the ball behind with a back set and surprise the opponents.
The back set may be challenging for new players. The setter receives the ball the same as she would on a front set. The initial body, arm, and hand position will be helpful in execution of the back set. Contact the ball near your forehead, but then arch your back and drive your arms high above your head. This extension should finish with your biceps very close to your ears. With experience and practice, you will gain confidence in setting to a player that you cannot see.
Good setters are able to set in either direction and have their form look exactly the same. So, if you want to perform the back set, don’t look back over the top of your head because the blockers will know what you are about to do and they will have advantage against the hitter.