You can also see that at 0:44 JiYeon is early going behind the mirror and she partly disappears. On the way out she more in time with the cut in video. Also, in order to pull this off, the camera must be in the exact same place or stationary. I have watched some live videos of Girls day, "Something", I downloaded them and I was able to sync them musically, and the robotic cams movement were identical in both songs, so it's more than possible to do this edit overlay.