This happened twice already, once 4 days ago, and the other time yesterday. This only works on Uncharted Region of China stage, from my understanding. Have Gold Lightan punch the ground, it happened both times on the right side of the stage around where the worker in the background is hammering a nail into the ground.
The ground will collapse and a new stage scene will appear, Gold Lightan and the opponent will land on a floating platform surrounded by water. It looks like a cavern, and the background scrolls. I would take a video of this but I can’t do this consistently, I tried for like 5 min in training mode, it has only worked in online matches for me.