The main idea behind the research is to use radio frequencies in order to transmit the secret data from the computer to the mobile phone. Mobile phones usually come equipped with FM radio receivers and it is already known that software can intentionally create radio emissions from a video display unit. Yes, from the computer screen. AirHopper demonstrates how textual and binary data can be exfiltrated from physically isolated computer to mobile phones at a distance of 1-7 meters, with effective bandwidth of 13-60 Bps (Bytes per second).
Read more here – http://cyber.bgu.ac.il/content/how-leak-sensitive-data-isolated-computer-air-gap-near-mobile-phone-airhopper