If you go to
System Settings > Workspace Behavior > Screen Edges you can theoretically configure "hot corners" so that when you drag your mouse to any configured corner it either displays all app windows, the desktop, or any other behavior of your choice. It's a bit like Expose on Mac OSX.
It's very easy to set up, but on Kubuntu 13.10 with my Dell XPS 13, it just doesn't work...or it would work once (but not for all corners!) and then will stop working completely.
I do have an external display but my laptop' screen is basically deactivated so that I only use the external display. Would be surprising this has something to do with it, though.
There's another question at Kubuntu 12.04, KDE 4.10, screen edges selectively working but I'm unsure this is exactly the same issue thus why I'm asking this one.
Is this happening for you too, and if yes, how did you debug it?