He explained it already. It looks for a ratio of number of users to posts. If your “small” instance has 5000 users and 2 posts, it would probably assume a lot of those users would be spam bots. If your instance has 2 users and 3 posts, it would assume your users are real. There’s a ratio, and the admin of each server that utilizes it can control the level at which it assumes a server is overrun by spam accounts.
Holy shit I haven’t heard that name in forever lol. Used to play it all the time before the 4chan raids