I keep doing this cool thing with the desktop, but ...

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http://www.kubuntuforums.net – ... I can't figure out how I did it. I'll have a bunch of apps open and Alt+Tab between them. Well and good. Then sometimes I'll accidentally hit something -- I don't know what -- and all my open apps are shown as squaes on the desktop and I can click on whichever one I want. Seems like a cool feature. I just don't know how I'm inadvertently accessing it. (HowTos)