Terry, what you've got there is a center rigger, not a greenstick.
A greenstick is used for longlining baits above the water, you rig a large bird and float to the end of the main line to hold tension then attach longline clips with baited hooks to the main between the bird and the boat, the greenstick keeps the baits suspended above the water until you pull down on the tag line and dip them intermittently. Once you get a fish on, the mainline breaks away from the greenstick and you retrieve it by hand (or often with a winch). It's very effective for yellowfin tuna from what I've heard although the practice would be illegal for sportfishing on our side of the boarder.
A center rigger on the other hand is exactly as you say, just an outrigger pointed up the center line of the boat to allow you to get another bait further back in the spread without tangling on turns. If I was running a bigger boat I think I would rig one, for me, I don't see the point in running more than the 6 rods I currently troll. Let us know how it pans out.
Cheers,