Ever since I was a kid, I noticed that whenever my nose was stuffy, breathing through my handkerchief seemed to instantly make me breathe normal again. It has happened each time without fail for years, so I know that it is not a random occurence that might throw me off.
1) The kerchief needs to stay really close to my nose, touching it. I tried covering it only half, and it still does work to a limiting covering area.
2) It usually requires a few folds to make that work. If I try it without any folds, it works only if I flare my nostrils. (Too many folds would lead to less air passing through)
3) Most importantly: the most optimal way, as I have found, is to keep an opening on the top (near the eyes) that let the air enter. If I tighten the kerchief, it doesn't work at all (that's why masks don't work for me).
It also works with merely covering my nose with my hand and letting the air enter from the top
My best hypothesis: I think the covering acts as a wind breaker. Which somehow helps. But merely breathing slowly (which might have a similar effect) absolutely doesn't work.
(This works for stuffy nose due to cold, but I don't know if it works for an allergy. I don't leave it covered while sleeping because when I tried it as a kid, I always bled through my nose; but that just might be because of a complication that makes me bleed through my nose more often than other people)
Also, can someone comment if they have noticed something similar? I don't know if this is a particular case.