Complex sentence: At the start of a round, the third tile from the back end of the dead wall on the upper layer is flipped and becomes a "dora" indicator, and its succeeding tile is recognized as "dora".
Split into simpler sentences: At the start of a round, the third tile from the back end of the dead wall on the upper layer is flipped and becomes a "dora" indicato. Then, its succeeding tile is recognized as "dora".