Spring-loaded Dock items, just like spring-loaded Finder items, are supposed to save you time by allowing you to drag a file over the folder/icon, hover for a few seconds, and have the application/folder open. Some people like these spring loaded items, while others don't -- with this Terminal "hack," you can either enable or disable this Dock feature. To enable it, open Terminal.app (located in /Applications/Utilities/) and type the following command, pressing enter:
defaults write com.apple.dock enable-spring-load-actions-on-all-items -boolean YES
To disable spring loaded items, replace the "YES" in the command above with a "NO."
Want more tips and tricks like this? Visit TUAW's Mac 101 and Terminal Tips sections.