I was just wondering about the effect used in the chorus of Judas Priest's song: a touch of Evil live. You can listen to it at 1:35
When Rob Halford sings the word "Evil" you can hear in the background a lower pitch singing the same word. Also it seems that there is a small delay.
I was wondering what kind of software can create this effect. Also how can a vocalist trigger such an effect in a live performance? Or is it the crew behind the scenes?
Sorry for too many questions, just curious to know more about this.