I do it too. I have never been sure that what you type actually made a difference but I usually type "spam" anyway, but the posts seem to go away even if I misspell.
The messages are viewed by twicker. The reason the posts disappear instantly is because a high post count user is reporting the post of a low post count user, which is almost always a forum veteran reporting a spam post from a newly created user. It also sets the account to limited right away too (prevents new posts, etc). If this is done on accident, twicker can always revert it.