Magical ring turns your arm into a track pad

Carnegie Mellon University researchers developed a system that turns your arm into a trackpad. Video demo above. From their scientific paper:

It consists of a ring, which emits a continuous high frequency AC signal, and a sensing wristband with multiple electrodes. Due to the phase delay inherent in a high-frequency AC signal propagating through the body, a phase difference can be observed between pairs of electrodes. SkinTrack measures these phase differences to compute a 2D finger touch coordinate.

SkinTrack: Using the Body as an Electrical Waveguide for Continuous Finger Tracking on the Skin (PDF via Wired)