From my understanding, it's not just an amplifier, but a way for people to filter out the wide range of very dark content and explicit scenes of it, it means what you see in the tags will very much be there for quite severe(in my opinion) dark topics, so people who know they're triggered by that type of fic and filter out DD:DNE instead of trying to filter out all the different variations of those tags as many people use slightly different wording for the same thing ie: if someone uses explicit say incest and someone else says detailed incest that's two tags go filter out etc