Ah, trapunto! I would guess from that that the Polydown Plus is denser, loftier or that it holds its loft better.
Hari Walner had a specific batting that she used for trapunto - can't remember the brand, at the moment. I remember that it was difficult to find compared to other battings and it's really stiff and springy.
You stitch this stuff to the back of the trapunto areas with water-soluble thread, just inside the design that you will stitch, then you cut away the excess and layer the quilt as usual. You quilt the outline on the design marked on your block, just past the edge of the trapunto layer of batting.
Is that how your book describes it, too?