My understanding is that not federating downvotes means that for blahaj communities, the canonical vote count has no downvotes, and instances can downvote if they like but their downvotes don’t get federated.
So if someone makes a post on this community and 20 people upvote it, then 10 Lemmy worlders downvote it, Lemmy.world would see a score of 10 but sopuli.xyz would see a score of 20 still.
Yeah I’ve since learned that this is not actually correct. I’m not sure where I got that from.