I made a post to funny@Lemmy.world, then in another tab I subscribed to science@lemmy.ml. Back in the tab I made the post in, the page updated itself to look like I made the post in science@lemmy.ml instead. I didn't refresh the page or do anything else... It just changed by itself.
What tutorial?
Your post's number is 123456, which is referenced as an abstract example in the "how to lemmy" intro thing I found when coming here