Keyboard junkies know that they can rely on the spacebar to trigger a Quick Look preview of a file, folder, or drive in the Finder. But what about trackpad junkies?
The good news is, there’s an option for the multitouch mavens, too. Perform a three-finger tap on any Quick Look-able item in the Finder, and a Quick Look preview you shall receive.
Repeat the gesture to send the Quick Look preview back into the abyss.
Source: Mac OSX Hints