It works like this: on the desktop side, SilverPush drops a cookie along with playing an inaudible audio, using speakers as audio beacons. The inaudible code is received and recognized on the mobile device by the software development kit that’s on the phone. This creates an immediate and accurate association between the desktop cookie and mobile device.
That's absolutely brilliant... and completely terrifying. It's just another example of what ad tracking companies will do given the technology.