Have you ever needed to look inside tons of folders? Or worse close a few hundred you have already opened?
For this trick to work need to be in the list view in the Finder. You can get there by clicking on the Finder icon in the dock to launch a new Finder window and then pressing ⌘ + 2 (command key next to space bar and the numnber 2).
Now select some folders by single clicking on them or select multiple folders by holding the shift key while you are clicking.
Don't forget while in the Finder ⌘ + a works to select all items in the current window.