The Hoya Variable Density filter uses two polarizing layers to control the amount of light that passes through the filter and into the camera lens.
At it's minimum effect the filter passes 1/3 of the light in a scene. That is equal to 1.5 stops on the aperture or shutter speed.
At it maximum effect the filter passes just 1/400 of the light in a scene.
That is equal to 9 stops on the aperture or shutter speed.

After the filter is mounted on the lens, turning the filter ring between Min and Max can greatly control the amount of light entering the lens.