According to the documentation, "Use this, this._parent, or _parent keywords rather than _root, depending on where your ActionScript is located." This allows movies to work correctly when they are loaded by other movies. For more information please see:

http://livedocs.macromedia.com/flash/mx2004/
    main_7_2/wwhelp/wwhimpl/common/html/wwhelp.htm?
        context=Flash_MX_2004&file=00000846.html