On Apple's keyboards you have the option to use the function keys either as F-keys, or for the functions Apple has assigned them by default. These include changing the screen brightness, invoking dashboard and expose, controlling iTunes, and adjusting the volume.
When the system is set to use the Apple-defined functions, by default the system will change settings such as volume level by a predetermined amount, and give a small click as an audio feedback for the volume change. While these default settings may be useful for most situations, there may be times when you would like to make finer adjustments to these settings, or not have features such as audio feedback.
The keyboard inputs support a few modifications to these adjustments if you hold down various combinations of modifier keys, some of which have more options than others. Just hold down the keys listed here and press the corresponding function key to get an alternative or finer adjustment to the desired setting.
Shift -- Changes volume without audio feedback.
Option -- Opens the "Sound" system preferences.
Shift-Option -- Offers finer adjustment of sound volume.
Option -- Opens the "Displays" system preferences.
Shift-Option -- Offers finer adjustment of screen brightness.
Option -- Opens the "Keyboard" system preferences.
Shift -- Performs expose in slow motion.
Option -- Open "Expose & Spaces" system preferences.
Command -- Shows the desktop.
Shift-Command -- Shows the desktop in slow motion.
Shift -- Opens the Dashboard in slow motion.
Option -- Open the "Expose & Spaces" system preferences.
No optional functions.
Option -- Immediately eject or open the optical drive (no slight delay), or open the secondary optical drive (if present).
Control -- Displays the power menu.
While most of these functions can be useful at times, the slow motion option was initially there for demonstration purposes when Steve Jobs and others would present OS X, and has managed to remain around. It offers no real advantage, and frankly it would be nice to see it replaced with or at least accompanied by an option to perform the respective functions at a faster rate. Despite this, it is fun every now and then to see all the windows moving in slow motion.