this code:
[you]
when used in a post will display the username of whoever is reading the post, making it seem as though you are talking directly to/about them.
Like so:
Guest is a fucking douchebag.
Which was actually created like so:
[you] is a fucking douchebag.
This trick can be easily spotted because it also turns the reference to the reader's username into a link to his/her profile.