Hierarchical menus were added to the Mac with the advent of System 4.1 on the Mac II and SE. Hierarchical menus are a human interface worry because they are horribly modal and require a lot of coordination in order to select the right choice consistently. For this reason their use is discouraged. Still, if you're going to use them, it's nice to know that the system software has been specially tweaked to make them as easy as possible to use.
First let's familiarise ourselves with the concept of hierarchical menus. Here's a picture of one, taken from the popular Mac OS application ClarisWorks.
The first cool things about hierarchical menus on the Macintosh is too dynamic to demonstrate here. [Hmm, maybe I could cook up a QuickTime movie one day. -- Quinn] If you slowly drag the pointer along the red line, the sub-menus Font, Size, Style, Text Colour, Alignment and Spacing will slowly flash up on the right hand side of the Format menu. But a cool thing happens if you quickly drag the pointer along the green line. The sub-menus don't do what you'd expect, which is flash up quickly and then disappear in a confused mess. Instead, the sub-menus don't appear at all until the mouse stops moving for a moment. It's not until you work on a system where this doesn't happen (such as early versions of the Amiga operating system) that you realise how important such a simple thing is.
The second cool thing about hierarchical menus on the Macintosh is even more subtle than the first. It's related to how you would normally choose an item from a sub-menu. Normally you would drag down the menu, locate the correct sub-menu and then drag across to choose the item on the sub-menu. Of course the required item may be anywhere between the top and the bottom of the sub-menu. If the item was at the bottom (such as the 48 Point item in the above picture) the natural action would be to drag along the green line. Fortunately, the people who programmed sub-menus on the Mac thought about this, and put some pretty cool special cases into the system. You can drag slowly along the green line and, even though you're pointer is no longer over the Size item, the Size sub-menu will continue to be displayed. But, if you deviate from the line, start dragging straight downwards for example, the Size sub-menu will disappear. Try it sometime, it's magic!