Help figuring out how a scene was created - Hausu, 1977


I recently watched the movie, Hausu (House, 1977) by Nobuhiko Obayashi and I was really intrigued by the mirror scene where Gorgeous’s face falls off in bits. I want to attempt to recreate it. I have been researching online to see if I could find a tutorial or an explaination on how Obayashi did it and so far all I’ve read was that he used a blue screen. I know it seems simple but I just can’t wrap my head around it. Could anyone explain or point me to something similar that help me understand how that scene was created?

Thank you!