Google has filed a patent with the United States Patent and Trademarks Office (USPTO) in which motion sensors built into a smartwatch expands its range of user interface feedback to the device's immediate surroundings. In practice, this means the watch can turn part of the user's arm into a 'mousepad' to select letters while typing and for other similar actions.
https://www.notebookcheck.net/Google-s-new-patent-could-turn-your-arm-into-your-smartwatch-s-keyboard.373631.0.html