What does this idiom story tell?

Idiom story tells the story of fire and fox's voice.

At the end of Qin Dynasty, Chen Sheng and Guangwu were arrested for being soldiers, but their trip was delayed by heavy rain, so they had to revolt. Before starting the army, Chen Sheng asked the fortune teller to calculate a divination, write "Chen" on white silk and stuff it into the fish's stomach. In the middle of the night, he built a bonfire with bamboo in the ancient temple, and learned the appearance of "Chen" as the fox said. In order to build momentum, he rebelled the next day and established the Zhang Chu regime.