With some ND filters, such as the Kenko PL Fader Variable ND3-ND400 you get a screw in knob that make it easier to adjust the filter. HOYA Variable Density, Kenko PL Fader, and Generic ND Vario 2-400 ...

On the back of the holder, you'll find an orange metal wheel to rotate the polariser to achieve the desired effect, and an orange metal locking dial to secure the holder to the 67, 72, 77, or 82mm ...