that's difficult to answer in full, and I don't think I'm the right person to provide a full history of it. I was around for it from reasonably close to the start, I think, but I was more of a reader/bystander than an active participant. but I can try to give my perspective on it

as a literary genre, ES began with twitter mini fics that usually depicted the perspective of an archetypal 'victim' or 'doll' figure in order to explore abuse, unhealthy, unequal and/or particularly emotionally involved relationships, and other related themes - trauma, codependency, etc. often, this figure was contrasted with another, the 'abuser' or 'witch'. a certain narrative voice was often characteristic of ES fics: poetic, symbolic, emotionally driven, melancholic, often lacking much dialogue and not providing straightforward emotional catharsis. over time, a rich mythology and symbology grew up around a lot of ES concepts, dolls and witches in particular. these were often initially personal to a particular writer, ...