A spiral apparatus is used to turn the object as the cutter is moved in one direction on the X axis. The result is a spiral cut.
The video below shows a number of things, including the use of a Spiral Apparatus (around 3:00 into the video).
This video is about the design, construction, and use of the Spiral Apparatus that he produced during the past year. It was given by Al Collins at the 2018 Ornamental Turners International Symposium.