Original Description:
Moving your mouse over the reveal button in this example will fade in an image that appears in the circle. Rolling off the button will fade it back out. This particular technique uses flag variables to determine which functions are run for a particular movieclip's onEnterFrame event (here, the movieclip being the image). This allows more than one function to run simultaneously within one onEnterFrame event as opposed to simply assigning the fade functions to the onEnterFrame directly.