HitchHike consumes 10,000 times less current than WiFi radios. It can operate for years on a simple coin battery, but the researchers say future versions might use tiny solar panels or even harvest the energy of incoming WiFi radio waves. HitchHike is a variation on what is known in engineering circles as a backscatter radio. It is actually more a reflector than a radio. HitchHike merely bounces WiFi signals back into the atmosphere – a signal that is known as backscatter.
Basically, a backscatter signal is when a radio wave passes through a receiving antenna, a current is produced. This current flow causes the receiving antenna to radiate the signal back out. If you were to view this antenna as connected to a load, the load can modulate the signal that is being received on the antenna. For example, in a simple scheme, the load could switch the antenna on or off by connecting it to ground or breaking the connection. More complex modulation schemes can be created – I need to read the paper to learn more!