From what I know, the instances share the posts between each other, but they need to have had contact with another instance somehow before they can get posts from there. Something like a user searching for an instance that isn't yet known to their "home" instance yet or following a link to it. As I understood it, this lets the instances know of each other. Posts of unknown instances won't show up on your instance until the connection has been made. So maybe a super instance could somehow include a newly created instance as soon as it has connected with any other instance already in the super instance.

(might not be too coherent since I know little about this all as of now ^^)