Here's how you can use the command key to open a folder in a new tab in the Finder in OS X Mavericks.
1. Open the Finder.
2. Find the folder you want to open in a new tab.
3. Press and hold the Command key.
4. Double click on the folder you want to open.
Your folder will open in a new tab window inside the Finder. This is a fast and easy way to make use of the tabs feature in OS X Mavericks Finder.
Read more: Tips and Tricks in Mavericks