There are many times in OS X when you will come across items organized into list, be it in Finder windows where you might have List View selected, or Mail messages that are organized into conversations. You might also find this in panels like the Finder info window’s various categories, or in places in the system preferences. If you regularly use these lists, then you might benefit from a little trick to help reveal or conceal items in them.
For many lists in OS X, you will see subcategories (e.g., Folders in Finder window lists) have a small triangle next to them to indicate whether or not they are open. Clicking this will turn the triangle down, and reveal the contents of the category; however, this will only open the first level of the category.
If you have multiple subcategories in a window, and especially if they are nested within each other, you can open them all by holding the Option key when clicking the arrow to reveal the category’s arrow to open it. This will expand the category and any that are within it, so you can see the hierarchy of items it contains.
This can be quite convenient for viewing a hierarchy, but may be more useful for ensuring all categories within a hierarchy are collapsed. For example, if you are perusing list view and upon opening or expanding a folder you see a large hierarchy of files expanded to reveal hundreds of items, you can manage this quickly by Option-clicking the triangle next to the parent category to collapse them all, and then proceed with opening them one at a time.
Along these lines, you can expand all the folders in a current directory by simply selecting them all (press Command-A, or Command-click specific ones to add or remove them from the selection), and then press the Right arrow to expand them. If needed, you can press Option-Right Arrow to expand the entire hierarchy of items in them.
Note that this approach is limited to List View in the Finder, and may not be applicable to other lists you see in OS X, but for most such lists you should be able to expand and collapse categories and sub-contents by clicking the associated triangle either with or without the Option key held down.