A worm drive mechanism can be very helpful for slowing down a gear train. Just two components can change the ratio drastically -- a task that would require a gear train with many more gears. This can also provide a great deal of mechanical advantage.
The end portion of the bit was modified using a Sherline micro-lathe and a Dremel tool with an abrasive cut-off wheel. We don't see how the gear itself was created, but it is modified with a Dremel and sanding drum to better mesh with the worm screw.
From the maker's description:
The wood screw, spurs and cutting edge at the end of the bit would need to be removed as well as the first flight of the auger, which had a shorter pitch. I ground away the bulk of the unwanted material on the bench grinder. The first flight was removed so the remainder of the auger would be of consistent pitch. The center shaft and inside areas of the flutes of the wood auger bit are not uniform. I saw making a helical gear as being a bit problematic for this purpose and decided to use a standard spur gear modified to run as a worm wheel. Since this would be used in a low speed low torque application, this simplified version of the worm wheel should be quite adequate. Because of the inconsistent form of the auger bit, a good deal of trial and error fitting was required.
If you feel like learning all there is to know about worm drives, check out The Theory and Practice of Worm Gear Drives (Kogan Page Science Paper Edition). As the title suggests, this book covers both theoretical and practical design aspects of worm drives. The excellent book Making Things Move by Dustyn Roberts covers the topic adequately for the average mortal, as does the affordable Basic Machines and How They Work.
If you'd rather just tinker a bit with a model (and I would, if I were you), check out Tamiya's Worm Gear Box. This worm gear can produce extremely low speeds of rotation. It has two selectable gear ratios of 216.1:1 and 336.1:1!
See more amazing creations by Ron Walters on his YouTube channel.