Every time a Wi-Fi signal is reflected off an object, the shape and makeup of that object affects the signal that comes back. But when Wi-Fi hits a wall, most of the signal gets reflected off the wall and only a faint bit of it reflects off the people on the other side.

To get around this, Wi-Vi transmits two Wi-Fi signals, one of which is the inverse of the other. When one signal hits a stationary object, the other cancels it out. But because of the way the signals are encoded, they don’t cancel each other out for moving objects. That makes the reflections from a moving person visible despite the wall between that person and the Wi-Vi device. Wi-Vi can translate those faint reflections into a real-time display of the person’s movements.

via MIT researchers can see through walls using ‘Wi-Vi’ | ITworld.