Seems like resting your finger on the mouse button is what toggles it between keyboard and mouse modes. It's an interesting idea, but I worry a little about the ergonomics of not letting the thumb "float" while the fingers are moving over the keyboard area. Normally, with a touchpad or trackball, you might keep your thumb on the buttons a lot of the time, but at the same time, you don't have to worry about the mousing function cutting out if you should let it move.