Imagine an ordinary left-right slider (line with a grab) which runs from say -100 to 100. The position is the value ... it's a slider.
I've sometimes seen rather: a horizontal area the same shape as a slider. It's marked with chevrons pointing to the right on the right and left on the left.
You put your finger on it (actually anywhere) and move your finger left or right.
If you move left, it decreases the value; if you move right it increases the value.
(Perhaps velocity sensitive.)
So indeed, a track pad really, rather than a slider. (Your movement cues a delta, you're not selecting a value along the length.)
a - Is this just something I've seen in my mind because I'm mentally unbalanced, or does this exist?
b - what's it called?
c - think of an app that does this?
Here's a scratch version I did of what's in my head ...
Or put it this way ...
Imagine your phone app has an area at the bottom of the screen, where you can nudge your finger left or right (ie, as if dragging), and that will affect some value. (Imagine it, say, tweaking one joint of a 3D model joint on screen, perhaps.) How would you "indicate" that area; that the strip is a (let's say) sort of "touch strip" ("mouse-like value adjuster strip") .... ???