
The trick behind with Siri Knudsen controlled piano is AirPlay. Among the piano is an Apple AirPort Express, which in turn is connected to the MIDI port on the Yamaha Disklavier piano. This Knudsen playing the songs on his iPhone through AirPlay automatically plays the piano. It does have one small condition: thus, the track played MIDI data.
The method is not difficult to reproduce, but you can still cost a hefty penny, not only because you have an Apple AirPort Express, but also the corresponding piano.