Mando,
It is a knob mounted on a threaded shaft that basically when tightened, forces the throttle quadrant closer together thereby creating more friction (like Gscholtz said) when you move the throttle back and forth.
When I fly with aircraft so equipped, I use it to make finer throttle, prop, and mixture adjustments when it is tightened and more coarse adjustments (larger movements) when it is loose.
