Edit: I guess there's this rolling method, which seems a lot more convoluted based on the videos.
https://www.amazon.com/Home-Comforts-Science-Keeping-House/d...
I think my parents' generation viewed duvets as unacceptably decadent, and thought all children should learn to fold hospital corners[0] to build character.
Also, many rooms let you get the temperature to ~15C.
</ducks>
Here is a video of someone actually folding the sheet as the parent requested: https://m.youtube.com/watch?v=ckTCocBCUN4
FWIW, my wife is tiny (160cm) and still does the reverse grip method
I just think of it like a really big pillow case. I put the pillow case on inside out so I do the same for the duvet cover.
I don't remember where I picked this up either, but I do remember it caused an ex girlfriend to get irrationally angry and tell me I was doing it wrong... that's when I knew she wasn't a keeper!
There was a program with Jacques Martin about "incredible" stuff. I remember a hairdresser who used a flame and J Martin almost agreed to try, another one about the world record in going back and forth through a door.
That one was the world record in how many duvets you can handle in a given time IIRC.
Note that this was 80, 81 or around that. This was the only source for such stuff in France so it was a big show (for children at least)
It was a bit more science leaning but got kids to awe just the same way.
my family and everyone i know do it the way @pablobaz describes it. it's simple and effective. change sheets whenever you feel like doing it, because its easy and fast... endof story
I grew up in a family that had (home made!) blankets and quilts, duvets were not a thing until I moved out on my own.
The shaking also has the effect of fluffing up the down/feather filling nicely and distributing it evenly, which you should do once in a while if you have a non-synthetic duvet.
The rolling technique described in the article just seems ridiculous, way too much work!