Using a specific network interface

Hello, I would like to know how I can force Mac OS X to use a specific network interface while connecting to my mac mini (from the iMac):
I'm connected to the internet (I mean the local network) using the airport, and I connected my iMac with the mac mini using an ethernet cable (because I want to transfer files faster, and I don't want to reboot my mini in target disk mode), but when I connect to the mini (using the "share" feature) it seems to use the airport connection. Of course I could disable the airport interface to force it to use the ethernet one (and it works) but I don't want to...

So, how can I force a specific interface? I don't mind using a command line command to mount it initially btw.

Thanks