In the Finder preferences there is a reference to 'Spring loaded folders and windows', and in the Secrets PrefPane there is an option for the Dock "Enable Spring load on all Dock Items"
What is it exactly that these refer to?
Here's a video of spring loaded folders in action on OS 8 (yes, OS 8, but the principle is exactly the same):
I am completely mystified as to why it has never been properly implemented in Windows. You kind of get a half-assed version of it in the folder pane of Explorer, but it's not the same as being able to 'spring' folders in the main pane.
They're things where you can drag something over them, hold it there for a second, and they'll open automatically. Some examples:
The paradigm crops up all sorts of places in Mac OS X. It's usually handy when you're moving things.