Quicklook has a hidden preference to enable “X-Ray Folders” like the one shown above. Once enabled, you can see small previews of the contents of a folder when you use Quicklook on the folder. All you have to do is run this command in the terminal:

defaults write com.apple.Finder QLEnableXRayFolders 1

And then run

killall Dock

Once the Finder restarts, try it out. It’s not incredibly useful, but it’s pretty cool either way.

To disable this behavior, just change the number at the end of the command from a 1 to a 0