Jog Wheel, Shuttle Ring
This device is used mostly for video editing.
The circle with a dent is the jog wheel.
The outer ring is the shuttle ring.
What is jog wheel?
The circle with a dent is the jog wheel. You can stick a finger on it and turn the wheel. Used mostly to move video frame by frame backward/forward.
You can use it to watch movie at slow motion with fine control.
What is shuttle ring?
The outer ring is the shuttle wheel. It has a spring that turns the wheel back to original position if no twisting force is applied. It is used to fast forward/backward video. How fast the video moves depends on how much you twist the wheel.
When your hand leaves the knob, it springs back to the original neutral position.
Jog/Shuttle wheel was popular with the VHS days. They usually are on the remote of expensive VHS players. Professional video editing hardware always have it, still.