ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Chapter 18. ActionScript Language Reference
Button.onRollOut( ) Event Handler
Flash 6; on(rollOut) supported since Flash 2
occurs when the pointer moves off the button (while the mouse is not depressed)
theButton.onRollOut() on (rollOut) { statements}
Description
The onRollOut( ) event handler is onRollOver( )'s counterpart; it executes when the mouse pointer is moved out of the hit area of a button while the mouse button is not depressed. As with onRollOver( ), onRollOut( ) normally is not used to create button highlight states, because visual button changes are created directly in the authoring tool. You should use the provided Up, Over, and Down frames in the authoring tool to create highlight states for buttons.
Example
cancel_btn.onRollOut = function () { trace("rollOut detected"); };