In use, the fixture is clamped in the mill vise.  A rotary file, held in a collet, is lowered into the slot seen in the other pictures.  The y-axis is used to advance the work into the spinning file a few thou at a time.  After each advance the work is swung back and forth between the stops to remove material.

If you make one of these, exercise care in its use.  Your hands will be close to the spinning collet and rotary file.  For extremely small pieces, I solder the part to a larger piece of metal to use as a handle or else use a small hand vise to grip the workpiece.