Okay seriously, how have I not known about this!? Is this from the latest game or has this been in the old games and I've somehow never noticed it?
From the rhythm game. The pose is from the song 純情Midnight伝説 and can be easily replicated with other idols, specifically replacing Yamato Aki from the further right position as seen in this: https://youtu.be/I-UTGKzpgLE?t=1m20s
Do note however you need to clear the song twice if you are new (Debut, Regular or Pro and then Master).