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

[carioca]

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

 

[koraks]

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

 

[carioca]

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

Thanks!