Shoot the devil in the eye

My wife spilled some salt, then threw some over her shoulder and said, "Shoot the devil in the eye".
Where did that phrase come from? How did she know he was looking over her shoulder?

It's an old superstition that everyone has a devil sitting on (or standing behind) their left shoulder, whispering evil suggestions, and an angel at the right shoulder, urging us to be good. (VSD)