DSLR Follow Focus
www.dslrsolutions.net/dslr-follow-focus
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
The DSLR Follow Focus eliminates the awkward perpendicular hand position required to turn a focus or zoom ring, giving the user much finer adjustment than could be achieved without such a device. The DSLR Follow Focus also works great for precise macro focusing and is equally at home on a zoom ring.