I can do that, as well, and it has a practical purpose when going hiking in the mountains or scuba diving. But nobody I've talked to about it understand what I mean, and I can't even explain what the technique is; I'm tensing of some muscles, certainly, but I can't explain how and which ones they are.
I learned to do it by isolating what made my ears pop when I yawn. I explain it as “kinda like yawning without opening your mouth” which seems to work.
This is how I explain it. When I first started scuba diving and explained to the instructor that it wasn't an issue to pop my ears, he was kind of horrified and was like no don't yawn underwater to do that. He didn't seem to understand, no matter how many times I explained it, that I can do it without actually yawning. You just like... mimic the start of a yawn. And then can continue it into a full actual yawn if you want to.
If it helps, I'm reasonably sure it's the soft palate muscles. To cause the effect you're describing, I visualize tensing a muscle that runs directly from one ear to the other, through the middle of my skull.
It's one of those things where trying to describe it to someone who physically can't do it or doesn't know how to may make them question your judgment and/or sanity.
