The red bar is actually a movieClip created on the fly using Flash MX's new vector drawing ActionScript.
It is also a method of myButton and is being called onRollOver. It is defined within the myButton Object using the this keyword

this.doHighLight = function() {



We will need a counter so initalize i to zero

i=0;


Now Create an empty MovieClip on the root timeline named highlight_mc underneath the menu item.

_root.createEmptyMovieClip("highLight_mc", 1);



Now that it exists set it's position, in relation to the Menu item that has been rolled over, to create the movie.

with (_root.highLight_mc) {
        _x = this._x-50;
        _y =this._y;



Define an EnterFrame event for the highlight_mc. The enterFrame creates a loop drawing a thick red line 200 pixels wide using ActionScript Vector Drawing methods. 

_root.highLight_mc.onEnterFrame = function () {
        if (_width<200) {
                lineStyle(20, 0xFF0000, 80);
                moveTo(_root.highlight_mc._x+i, 10);
                lineTo(_root.highlight_mc._x, 10);
                i += 10;



Once it is 200 pixels long clean up the enterFrame so we don't have a neverending loop to slow down the Flash player

} else {
        onEnterFrame = undefined;



Now finish up the Code Blocks

}// end if-then-else
};// end enter frame
}// end with
}; // finish the function