It becomes a ranked minigame where you play the songs in the game on the piano system in a rhythm-based way, using the analog sticks. And it's very tricky. I could only manage a B rating on what I believe is the easiest track (going by what I've been told), and you need A's on all six tracks to get the trophy. Weirdly you get literally nothing for achieving a B rank, so it's weird it's even there.