ActionScript for Flash MX: The Definitive Guide, 2nd Edition By Colin Moock
Chapter 18. ActionScript Language Reference
MovieClip._ymouse Property
Flash 5
vertical location of the mouse pointer
read-only
mc._ymouse
Description
The floating-point _ymouse property indicates the vertical location of the mouse pointer's hotspot, relative to the coordinate space of mc. If mc is a main movie, _ymouse is measured from the Stage's top edge. If mc is an instance, _ymouse is measured from the instance's registration point. To obtain a consistent _ymouse coordinate that is always measured relative to the Stage, use _root._ymouse.
Example
Placing the following code on a clip causes it to follow the mouse pointer's vertical position, relative to the Stage (the clip moves up and down in a straight line):
onEnterFrame = function () { this._y = _level0._ymouse; }