It's not really a trick, it's that your posts are saved as drafts while you work on them. This is account-level data, not a cookie, so it is available anywhere that you are logged in. The floppy diskette icon in the toolbar above the text input is where you can manage drafts. You might want to delete one, for instance, if you started to make a really vile post, and then changed your mind about posting it, but it comes up every time you try to interact with the thread you started to post in.