A way to apply forced-edge printing to books?

I am a writer and while I have Adobe everything, I use Vellum to format my ebooks and print books because it's just faster and easier.

They just added a feature to their software to add forced-edge printing to print book formatting. Basically, it involved applying a .pdf file with the full bleed edge design and the Vellum software combines the files to make a book with printing on the outer edges, visible when the book is closed.

I'm NOT talking about aftermarket fore-edge printing techniques; I'm talking about a .pdf file that adds small slices of the design to each edge during the printing process. This is becoming very popular now with POD books, and obviously it's doable. Someone cracked the code. There are a couple of online sites that will do it for a fee, and there are sites offering the actual design files ready to go, but those are pricey. (And I am both broke and stubborn.)

I am not a programmer. However, what I "think" is happening is that there's a script or something allowing the ability to specify the number of pages, and then "slice" the image a certain way on the full-bleed edge so it's parsed among the pages on the edge of the .pdf

I have been scouring the internet for tutorials and have found nothing. Meaning yes, people have figured this out, and yes, they don't want to talk about how they do it.

Here's an article (the author is a friend of mine) who talks about the process, but she also doesn't understand the "how" portion of it.

https://www.zoeyork.com/blog/pine-harbour-is-getting-a-nice-warm-cozy-flannel-update

Help? And thanks in advance.