Imagine this way. You have three papers stack together. The top paper is the duvet. The bottom two paper are the cover. If you flip the bottom page over the other two papers, you have the duvet paper between the cover pages. For the real cover, you can not do that because the three sides are stitched together. So you roll the them together then you can invert through the opening.
Thanks for the explanation. I’m with you up until that last part. Maybe it’s just a “you have to try it kind of thing. Otherwise I’m not really sure the mechanics of what is happening when the unrolling is happening.
