Why have one screen when you can have two? That seems to be the reasoning behind the KF600's design. The top screen acts as a normal display, where you can view all of your phone's content, and the bottom one serves as a navigation pad.
Unlike a normal navigation pad that stays static, though, the KF600's changes depending on what you're using on the phone. If you're in the menu, then navigation keys pop up. If you're using the music player, then--you guessed it--music keys appear. Click here to see more.
(Source: Crave UK)