Yes, closing your sinuses is a natural reflex response for humans, and people have greater or lesser at will control over it.

The nose holding for swimming is more about how strong that sinus closure is and endurance. People with larger sinus openings have a more difficult time keeping them closed and resisting pressure like water entering from jumping into a pool. Also some people have a hard time keeping them closed for any prolonged period.

In other words, you just have totally ripped sinuses breh.

face claw

Did you not read the "How Not To Give Away You Are A Lizard To The Humans" pamphlet?

You might be part otter. They can close their noses from within when diving.

Yes you can, but it's not very strong. So when jumping into water, you might want to force close with fingers to prevent pressured water to get into the nose even if you keep it closed with your internal muscles.

I almost broke my timpanus due to a very high jump without using the fingers

Shut from where?

Shut your nostrils completely? That doesn't seem normal.
Shut the air passage near where it enters your throat? I think that's normal.

How else do people blow out birthday candles, and blow up balloons? The air could just escape through the nose!

I think with diving and other situations is to prevent the exposure of the inside of the nose. Especially when jumping in the water and and pressure can be much higher.

I can’t do that. I never felt the need to hold my nose in any of the scenarios you described. I am pretty sure when it comes to your nasal cavities there are no muscles involved.

You can what?

I don’t think I can do that. I can lock it down further back?

Do you, by any chance, have slight trouble breathing with a light cold, get a clogged nose easily or even have reduced sense of smell?

Because I do and I can also do what you describe.

Yes I can stop myself from sneezing without holding my nose. But I've heard it's bad for you. Some people cannot do it, and are surprised when I do.

I mean, in all of those circumstances I just hold my breath. I'm certainly not wanting to breath in a bad smell through my mouth.

But if the question is can I elect to breath through my mouth without pinching my nose, the answer is yes. I wouldn't describe it as "shutting my nose," though, so maybe we're not talking about the same thing.

Yes, but then you're just eating the poop offgas when you clean.

My great grandma used to say, "I'd rather smell the smell than taste the smell!"

No but I can close the air duct just above my soft palate (not really sure what that part is called), so effectively I can't breathe through my nose but if I were to go underwater water would still get in, if that makes any sense. I'm not sure if this is common. I know that that part is supposed to close automatically while swallowing to prevent food from going up in the nasal cavity, but idk how common it is to be able to control it separately. It's handy to avoid smells without using my hands though :)

We need video proof

I can shut my nose mostly, but when diving into water from a significant height, it will always shoot up my nose, so I plug it with my fingers.