Hi Dame Melusine,
I wouldn't recommend floding your sinuses. IME it leads to bad things.
If I'm not wearing a mask the usual way I stop flooding up the nose during flips or when swimming upside down is to expell a little stream of air through the mpse to prevent water coming in. Of course, this reduces your underwater endurance. The other way is nose plugs or a nose clip, which is what freedivers who do line diving use when they are not wearing a mask.
When you're just going down the ordinary wayusing a dolphin kick, there's usually no problem, your lungs balance the rising air pressure so that no water comes in. I'm sure a lot of your questions will be answered during your freediving course.
If you want to ask really technical questions and get advice from Canadian divers, you could consider joining the DeeperBlue ( https://www.deeperblue.com/ ) forums https://forums.deeperblue.com/
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