Tom,
The way he explained it was that the file or rasp is subjected to a "liquid honing". Some kind of fluid that has abrasive in it. The abrasive removes a small amount of metal from the gullet of the file tooth, up to the top edge, thus creating a sharp edge of the same profile. Kinda like sharpening a broadhead - removing metal to the edge. Over time, there won't be enough there to work with and you replace it.
He did say that some damage of files or rasps is irreversible but that most of the surface might be salvageable. He also told me that some cheap tools are made with a thin case hardening, today. Those won't take to this method but he will inform the customer of that. We have seen this in some tools, particularly inexpensive hand saws and files.
He has much more for sale and services more than files and rasps. Rotary tools, end mills, diamond coated cutting tools, etc.
Ray