Google ATAP: Welcome to Project Soli [video]
youtube.com
youtube.com
> Project Soli is developing a new interaction sensor using radar technology. The sensor can track sub-millimeter motions at high speed and accuracy. It fits onto a chip, can be produced at scale and built into small devices and everyday objects.
I suspect they could embed this behind the screen of a smartwatch, and allow broader motions in the air above the watch to select object on the smaller watch face. This in particular as it can detect 3D location, so it can tell a press (moving closer to the screen) from a selection (horizontal and vertical motion).