I've heard of it before but also looked it up again:
https://petapixel.com/2014/10/19/ada...egatives-hand/
It's basically a light box that vibrates the negative put on it, so that the retouched bits by the photographer / retoucher are a lot more random.
And here's a link to a 'before and after' retouched picture of Joan Crawford, from time gone by = amazing!:
https://petapixel.com/2014/10/17/bea...-photoshopped/
Terry S