I think I understand Fire's problem. You do want to be able to move your rib cage. Ideally, I think, the rib cage should be making a figure eight also, but in the opposite direction as the hips--when right hip is back, left side of rib cage is forward and so on. All that, while keeping your shoulders level and your head still.
To practice that, I just do that. To isolate the ribs, first try standing with your hips still and move your rib cage to the left and right and forward and back, then one side forward and one side back. Get your back muscles involved. Then you can work up to figure 8s. I do hip figure 8s in isolation, then rib cage figure 8s in isolation, then figure 8s with both (in oppostie dirctions). Isolating the ribs is hard, but it does get easier with practice.