If there were no other springs, then wouldn't it be like a solid axle (non-independent suspension)? It would only compress and rebound equally on both sides. If there was a deflection up on one side, there would be a corresponding force down on the other side. I think the pivot points where the rods attach to the frame might have some spring or tortion bar attached to them that we can't see. It is just a total guess on my part though. The main spring in the middle would work for both equal deflections and to compensate for downforce (guessing again).