Instant Tethering links up devices that are associated with your
Google account to make it easier to stay online when a connection drops. So, if one device loses connection, it can just ask if you want to tether to your other device—no additional setup necessary (uses Bluetooth to negotiate the connection). It even shows you the battery level of your other device. This makes the most sense with a
tablet connecting to a phone, but it should theoretically work with any devices as long as one has a cellular connection.