There was an HN post a few weeks ago about Mercedes putting something in their car that activate the reflex just before an accident. To protect against hearing loss from the loud crash of an accident.
I always assumed everyone could do that - I've been able to do it for as long as I can remember, and can (to a minor extent) vary the intensity between ears.
I often get the same experience when say on a train on a summers day. It's like an on/off switch, rumbles in the light, quiet in the shade.
Just thinking about it now ... I kinda pull my jaw back a bit and push my tongue against the back of my throat, like the first half second of a swallowing action. It's enough to make a little 'click' in my hearing, even when I've not changed altitude.
How did you learn this?
Do it slowly, but at one point, you'll need to force a little to trigger it.
Then the ear will go back to it's original state.
Do it several times. At one point you won't be able to do it anymore. The ear will stay like that for some time. Rest a bit.
Then practice again.
Once you got it, force yourself to yawn, and try to reproduce the effect while yawning.
The once you can do that, try to do it without yawning.
In the end, it's not a useless skill. It's very useful when you take the plane a lot, go skying in hight altitude or dive in deep water. It let you release the pressure from your ears at will.
We should teach it to all children. You often see them crying in planes because their ears hurting while the pressure changes. If you encounter that, ask them to do the first exercise (blow with the nostrils blocked). If they can't (sometime it's too late, the pressure is too high and that would hurt), make them chew something.