Thumbwheel	

    The small transparent box that normally appears at the top of
    the Vertical Scroll Bar and on the left-hand side of the
    Horizontal Scroll Bar is called a Thumbwheel.  The Thumbwheel is
    used to help you efficiently scroll vertically or horizontally
    through a document.  To move the Thumbwheel, move your mouse
    pointer to the Thumbwheel, PRESS AND HOLD DOWN the Select Button
    while moving the Thumbwheel up and down or right and left along
    the respective scroll bars.  The movement of the Thumbwheel
    corresponds directly and in proportion to the movement within a
    document in a window. 

    This action applies for all windows except for Vertical
    Scrolling in the Document Window.  In the Document Window, the
    Thumbwheel is used for paging vertically up and down or forwards
    and backwards within the document.  To move forward or backward
    one screen at a time within the current document in the Document
    Window, move your mouse pointer above or below the Thumbwheel
    and click.  (See Scrolling - Horizontal and Vertical in this
    Help system)