In the room immediately below the dwarf kingdom, theres a little pool. The second door is behind a waterfall. 6 spaces down and 1 space left of this is a single tile notch in the wall. Stepping onto this makes the screen flash blue.
It only works once per entry into the room, and seems to do absolutely nothing. It certainly doesnt restore HP/MP or anything, despite flashing in the same manner such healing things do.