The simplest overlay defines four configurable virtual keys at the top edge that, from right to left, default to a key that disables the "numpad mode" (this returns the Touchpad back to its entire surface being a pointing device), a key that launches the OS X calculator, and the keys F17, and F16.

Below those virtual keys are more that make up a numeric entry and basic mathematical operations. The remaining pad real estate can be used as normal to control the mouse. The other overlays add more programmable virtual keys as well as a number of fixed keys for things like cursor control and editing functions.

On the simpler, more compact, layouts the areas not taken up by virtual keys still allow for normal mouse control but, because you have to disable the single and multi-finger tap gestures (done through the OS X trackpad control panel applet), you lose some normal trackpad functionality.

You can leave the configurable virtual keys to be whatever their default is or select them to act as a predefined function key, launch an application, or execute a keyboard shortcut (such as CTL+V or CMD+Del). You can also write on the overlays so you can remember what the configurable virtual keys are to do.

My only complaint with the entire product is that the overlays are printed in white on clear film so while they look consistent with the whole Apple aesthetic they can be hard to read in some lighting conditions (white on silver doesn't have much contrast). However, get your lighting right and this is a cool hack. For $30 the Mobee Magic Numpad gets a Gearhead rating of 4 out of 5.