Please bear with me as I attempt to describe this control (since I don't know its name I wasn't able to google image search for it).
It's a physical control, used for adjusting a quantity up or down. Imagine a disk, maybe 1/4 inch in thickness, with notches around its circumference at regular intervals. Now take the disk and, holding it perpendicular to a control board (the same way a car wheel is perpendicular to the road), imagine sinking it into the board until only a little bit of the disk is still visible, sticking out if the top of the board like an iceberg out of water. You can now use your thumb to rotate the disc in place by pushing on the part sticking out. This rotation changes the value of whatever quantity you are controlling.
What is a such a control called?
Have you seen skeuomorphic versions of them used in mobile apps, and what are the usability issues around them?