To use Quick Look on an item in Finder with Mountain Lion, you could use a contextual menu, hit the spacebar, or press the keyboard shortcut (Command-Y), but here’s any even easier way. If you have a Multi-Touch device, like a Magic Trackpad, now you can just do a three-finger tap on the file to open the Quick Look window. To close the window when you’re done, simply tap with three fingers again.
For those who don’t like this little trick, you can disable it in System Preferences -> Trackpad and uncheck the Look Up option. But note that the Quick Look functionality uses the same gesture to look up word definitions so if you uncheck this box you’ll actually be disabling both.
Source & Image Credit: The Mac Observer