I think that what imaginenore is suggesting is that if you are communicating between your public network-connected machine and your air gapped machine, there is no longer an air gap (barring some very specific use cases). The technique you're describing is actually an attack used to defeat security of machines not on untrusted networks[0].

[0] https://en.wikipedia.org/wiki/Air_gap_malware

EDIT: This is specifically the method I was thinking of: http://www.jocm.us/index.php?m=content&c=index&a=show&catid=...