I found this config option at http://www.browserstack.com/automate/capabilities-and-timeouts
I tried to set it to false, the tunnel was still working. But I noticed after several tries that it was only updating the Tunnel property (a boolean) in the test session report at https://www.browserstack.com/automate ! I'd say it's just informative.
I believe they are not detecting and/or updating the tunnel state in the session report (which is a bit weird, but who knows what the hell they are doing ^^).