Fake Format is, as its name fairly shows, an application which will make other people believe they are having all the files removed from their HD.
At first, it will show a Dialog box where it tells that the HD is about to be formatted in 13 seconds unless you press the 'cancel' button, but that button is moving around the window.
After that, the usual MS-DOS screen will appear on screen and your friend will only be able to close his eyes while the catastrophe takes place.
At the end, Fake Format will show a message telling that it was a joke. it is so funny, isn't it?