One of the features of the OS X Dock is the ability to magnify the items in it as the cursor passes over them. You can control this feature in the Dock System Preferences panel, where you can set the amount of magnification and enable or disable the feature. (You can also enable or disable it by Control-clicking on the Dock’s divider bar, and choosing Turn Magnification On (or Off) from the pop-up menu. You can’t control the amount of magnification from this menu, however.)
Leopard adds a new feature to this magnification effect—the ability to temporarily reverse the on/off status by holding down a couple of modifier keys. To demonstrate, first run your mouse over the Dock and notice if the icons magnify or not. Now hold down Shift and Control, and then run the mouse over the dock while still holding down those keys; you’ll see the opposite of the prior behavior. In other words, if you’ve got magnification turned off, you’ll see magnification effects when you hold down Shift-Control. If magnification is normally on, then you won’t see the effects when holding down Shift-Control.
If you keep magnification off, you might wonder how to control the size of the effect when you use this temporary override. The answer is in the Dock System Preferences panel: just temporarily enable Magnification, drag the slider to set the desired amount of magnification, and then disable Magnification again.