Topology of fitting duvet covers

When I was still a teen, grandma showed me a very clever way to fit a cover onto a duvet. She tried to teach me but I didn't manage to do it at that time. A few months ago I was fitting my duvet with its cover after laundry, I thought really hard of it. With a lot of thinking and practice, I finally did it and now I write it down here in my own words.

1. Spread the cover over a bed inside out.
2. Put the duvet on top of the cover, align the four corners.
3. Starting from the far end, roll the cover and the duvet together toward the opening edge of the cover. You will end up with a roll of duvet laminated by the cover on the outer side.
4. At the opening edge, grab the two corners of the duvet at the aligned corners of the cover from INSIDE.
5. Stuff the edge of the duvet bracketed by these two corners into the cover.
6. Repeat rolling the laminated duvet all the way till you reach the opening edge of the cover again.
7. Grab the 2 corners, spread the duvet and you will have the cover fitted tight at the corners.

It was magic to me when I first saw grandma did it. After all the years I figured she had solve a 2-dimensional problem by 3-dimensional topology! A few months ago when I finally got to do it on my own, I asked grandma who taught her this. She said she figured it out herself.

I always tell people grandma is very smart.