Thanks @mrelwood for the detailed reply and info.
Okay, I think I finally have the right mental model here. I'm gonna get this down from a slightly different viewpoint, in the hopes that the few beginners who didn't follow that explanation will "click" with this one, and go from beginner to skilled in a shorter time.
- - -
The thing about EUCs (and hoverboards, which I started on) is that they not only try to keep themselves under your center of mass, but that they also have faster reflexes than humans do.
Why does this matter? Because when most people are learning, they try to keep the thing underneath themselves by consciously moving it into that position, which never works. You see this best on a hoverboard, which is side-to-side stable (because of the two wheels). Instead of falling off to the side, they wind up jerking back and forth ("vibrating") then jumping or falling off. It's a feedback loop caused by the fact that the rider and the device are both trying to keep vertically aligned, but they're fighting each other. Once the beginner learns to just assume that the hoverboard is a stable flat surface that they can just step on a trust will stay in the right place, the fighting stops, the jerking stops, and the forced dismounts stop. The device and the rider have made peace with the idea that the device, not the rider, is in charge of keeping everything vertically stable.
Okay, but what does this have to do with sensitivity? Simple: if you remember that an EUC has (way) faster reflexes than you, then you'll see that a high-sensitivity device will respond to a forward or backward lean by scooting underneath you quickly. More quickly than your body probably expects at first. So what starts as a slight lean instantly becomes you standing straight up on your device, because the moment you start to lean, it just gets right under you. A low-sensitivity device, on the other hand, will respond to your lean less quickly/aggressively, allowing you to maintain or even increase that lean before it adjusts its position to get under you again.
This is where my kinesthetic intuition misled me: with a high-sensitivity setting, I'd lean and find that the device wasn't accelerating easily. In retrospect, that was because I'd lean, the device would get under me, and then suddenly I wasn't accelerating anymore because I was no longer leaning. It felt like I had to lean hard to keep moving, which I guess was actually accurate: it's harder on a high-sensitivity device to continue to thwart its desire to get under you (and thus return you to stability and vertical alignment).
On a lower-sensitivity setting, I'm able to more easily maintain a steeper lean, which ultimately leads to more acceleration.
What at first seemed counter-intuitive ("if it's 'more sensitive,' why can't I *go*?") is now intuitive, because sensitivity really means "how aggressively does the device try to stay directly under you?"
With that new understanding, I feel like I now better understand what the EUC is trying to do, which increases my sense of control and comfort, and makes it easier for me to understand how adjustments to the sensitivity setting will actually affect the feel of my ride.
Thanks all. Hope this helps at least one other person.
