In 1995 Tom Zimmerman and Neil Gershenfeld realized that the weak
low-frequency fields that the Physics and Media Group were using for sensing
contained information that was being used to separate them from background
noise. Instead of otherwise ignoring this information, it could be modulated to
send data through the body to create a Personal Area Network (PAN). This means
that personal information stays with the user where it belongs rather than
being radiated out to the surroundings where it could cause interference or be
eavesdropped, and that physical gestures can contain logical meaning so that a
handshake could exchange business cards, a hand on a doorknob could unlock it,
or fingers touching a keyboard could transmit a cryptographic key.
Here is a link to a paper explaining how it works.
This MPEG clip (16MB) shows the PAN system in action.
This photograph (linked to a larger version) shows a shoe made by Rehmi Post
containing a PAN transmitter (pulled out for clarity), and in the foreground
is Matthew Reynold's PAN III RS232-to-body modem.