Ok, basically it's like this...

You make a post titled "Dude, I just totally stole candy from a baby!" and you make the post f-locked. I, in turn, write, "Dude! That's totally awesome! I stole candy from a baby last week too! It was a chocolate bar!"

Then, I click to share it on Twitter and Facebook. That means everyone who is connected with me there gets linked back to my comment. While the post itself is still locked (They can't read what's in the entry), they can read your username, the title of the post and my comment. And based on what my comment said, it can give away what you posted about.

And, if someone uses your name in the comment, then it wouldn't take someone that much of a guess to find out who you are in RL. And like I told Laura, I like to control the line between RL and fangirlness.