You don't see anything if someone doesn't subscribe to it.
If you follow someone on mastadon from lemmy/kbin in theory you'll get.. Something. Not sure how it works exactly. If threads federate then the same will be true of threads users.
How do you follow someone on Mastodon from Lemmy? I'm not sure that's possible. I see you're a kbin user so it's possible for you, but I don't think Lemmy users can.
If Lemmy instances defederate from Threads, it'll just be a symbolic message that the fediverse is anti corporate greed. A good precedent to set imo, but our actual experience probably won't be impacted either way.
What really needs to happen is Mastodon defederating from Threads.