This mouse has a ball on the bottom, but it doesn't always need it.  There's a little button underneath that, when you press it, will send a signal to the base unit about where your mouse is in the air as you move your arm around.   The gyroscopic function works okay for presentations and the like (which I suspect it was originally designed for), but it's not really precise enough for everyday use.  That's really a shame, what with how much it seems to impress other geeky guys.  If only it impressed women.