You can get this in conjunction with the High Scorer achievement.
Simply play a song you are familiar with on Expert. When your Dance Gauge is full, tilt RS in any direction. Repeat until finished. I should note that you can activate this as long as you have control of the game - this means you can activate it at the end of the song, even if the arrows have stopped scrolling. I did this on "So Fine", and got 1.6 million points.