Direct positive paper, what's the science behind it?

Out of curiosity, I have always wondered what the science behind direct positive paper is, ex. Harman direct positive paper.
Is there any photo chemist or knowledgeabled member here, that could roughly shed some light on how this process is achieved?
Thanks

This may be of interest: https://www.photrio.com/forum/threads/direct-positive-paper-how-does-it-work.68539/

koraks said:

This may be of interest: https://www.photrio.com/forum/threads/direct-positive-paper-how-does-it-work.68539/

Click to expand...

Thanks!