Bart

The Z-axis is used to detect whether your X-plorer is laying flat/horizontally on the table or if you're holding it vertically while playing it. The guitar controller manual on page 3 says you can lay it down flat if you're having trouble with the motion sensing while you're trying to navigate through menus.

Looks similar to Datelens: http://www.windsorinterfaces.com/datelens.shtml