Possible they are playing to a click, in which case the backing vocals are already there. But personally, I think it's more likely they are triggered by Rudess, Mangini, or a tech/sound guy. Rudess could do it through any of a number of his gadgets. I forget what the thing is that Mangini uses, but he has a gizmo (or a few) on his kit that does samples, so all he has to do is hit whichever pad has the sample loaded up to trigger it. Could be him. Or could just be a tech offstage doing it. That is common as well. Either way, it doesn't bother me. One of the first things that came to my mind when I found out Portnoy was leaving was that they need to find a drummer who can sing, because they are losing a backup vocalist, and I was worried about the vocals sounding thin where they are supposed to have backing vox. If they are triggering the vox, they're covered, so I'm happy.