Wi-Vi works by emitting Wi-Fi signals at a wall. Some percentage of those signals penetrate the wall, reflects off people on the other side and then returns through the wall to the Wi-Vi device. The Wi-Vi array emits two types of signals that are the inverse of one another. If both signals hit a still object, such as any furniture or the wall itself, they cancel each other out. If they hit a moving object, however, the canceling effect doesn't occur and the moving things show up in the Wi-Vi.