Two of the key limiting factors of wearable tech are battery life and the manipulability of the interface. If something is small enough to wear unobtrusively, how is it possible to physically manipulate it so you can control it? Your fingers, after all, are not the finest instruments on earth.
Chirp uses sonar via an array of ultrasound transducers—small acoustic resonators—that send ultrasonic pulses outward in a hemisphere, echoing off any objects in their path (your palm, for instance). Those echoes come back to the transducers, and the elapsed time is measured by a connected electronic chip. When using a two-dimensional array of transducers, the time measurements can be used to detect a range of hand gestures in three dimensions within a distance of about a meter.
The control is hand motion, not finger pointing. At least not yet. But think about pad or phone multi-touch interfaces that don’t actually need touch, and you’ll start to understand how neat this could be.