I used MouseKeys on and off, sometimes quite a lot, before switching to Datahand.
There is an explanation in (Windows XP) Help and Support Center, type MouseKeys in the search box. However, it is pretty mangled since it, like most of the material in Help and Support Center, was automatically extracted from .hlp/.chm files. I once managed to open the original file, but have totally forgotten how.
In any case, the eight keys surrounding the 5 button move the mouse in the corresponding direction. Clicking is accomplished by pressing 5, double-clicking is +. For selecting and dragging, button make is 0, button break is . (period). There is some internal state that determines which mouse button is currently mapped these keys, initialized to left button. This state is changed to right button with -, to both buttons with *, and back to left button with /.