The rules...

1. Every tweet on public profiles are ... public. Anyone can read them but not all your followers see all your tweets (more later).

2. If your tweet starts with an @mention (as in, the first character of the tweet) then only the user you @mention will see the tweet in their feed (as well as anyone else you @mention in the tweet). It does not display on your followers feeds. That is, unless, you both have mutual followers.

It's a hack and many people don't understand it.