they're not really submitted by a mod, just held until they're sure it's nothing illegal.
Think of it like shipping something to a friend across the border. You send it, it's all your info, but customs has to look it over. If they determine it's fine, then they send it on to your friend (or in this case, viewed to the board), if they see that it's something illegal, they generally send it back to you, or at least give you notice that it's been derailed, and that it's not getting put up. They also have a tendency to warn you.
I have to use it occasionally in a couple of boards I'm admin on, mainly though, I use it in good fun with other mods/admins. :p
*Jaaku waits for this post to be submitted