Switchboard allows the extremely disabled to use an Android device with a single button or switch. This means that in principle any muscle with reliable voluntary control can be used for web browsing, sending messages, or anything else, for example by using a foot pedal, tilt switch, bite switch, tongue switch or sound-operated switch.
The text entry interface highlights different areas of the keyboard in sequence, and the user activates their switch/button at the appropriate time to narrow down on the key they wish to press. A predictive text model (trained on a hundred million words of text) attempts to provide intelligent suggestions. Most words can in practice be typed with 5-10 presses of the switch/button.
Web app version
The precursor to Switchboard is a web app called Roger, which implements a scanning keyboard for disabled people to type with.
The source code of Roger (working, tested on a few systems, but undocumented) can be downloaded from the roger github repository.