Where is the shortcut on The Amphiseum (Reverse) Located?

Near the end of the track (just a bit before the big drop-off) there is a hilly, electromagnetic section of the track that ends in an upward slope. Right after the upward slope bit, you'll notice a small white square off to the top right of the subsequent tunnel entrance.

As you hit the end of the upward slope, simply tilt the front of the ship up, use a boost, and aim a little to the right to go straight through the holograph square; you'll come out JUST before the big drop-off.