Things here get pushed from publishers to subscribers and their servers, and boosting is basically a way of republishing things.
Communities are, in some significant way, bots that boost anything that they see, and they see anything that mentions them, or that is in reply to anything that mentions them. Lemmy and kbin just hide the "To:" field of thr message, which is where the community bot (or "group actor" in Fediverse technical lingo). Boosting things that mention the community bot also triggers the not to boost it in turn, which sends it out to everyone following the bot.
keegomatic