Final word gives you the opportunity to have one last pitch to you're visitor before they leave you're website.
Your visitor can not leave your website without hearing your final word. If they attempt to hit the browser back button, or type in the address bar, or if the mouse leaves the main window at all, final word is triggered.
You can set the stack to only be shown once to each visitor, or aggressively show it every time they try and leave the website.
Put what ever you like inside the final word popup.