I didn’t see this word used in the video, but the technique with the polishing strings is called, thrumming. (probably my favorite word in jewelry making)
Another thrumming idea for pre-finishing (before polishing) is to use dental sanding film. I got some from a dental supply company, but you can get it from Amazon or other sources. It’s very fine sanding grit on thin plastic film. You can use the film as is or cut it into narrower widths. It’s kind of gross to think about, but it’s designed to sand between teeth, so it goes from fine to super fine. You’d use it the same way you use the polishing string.
Here’s an Orchid thread that talks about the dental sanding film along with other sanding/polishing tips.
Jeff