Knurling a knob using a tap - GIF

[Frank S]

Looks more like hobbing a worm gear using a tap. When using this procedure to make worm gears it is better to cut a grove around the center of the blank then start the gear cutting process at the end of the taper on the tap This will preselect the pitch diameter and final depth of cut as the gear rotates and the start of the tooth profile is exposed you move the gear blank towards the main body of the tap will make cleaner teeth and will not be recutting a tooth
Doing it the way it was shown in the video at least 1 tooth is removed by the time full depth is achieved