Where did Wong Tze Wah laugh at "inventing" backwater?

The first appearance of the word backwater is the opening part of the play, and the second appearance is the Xie Chang part of settling accounts before autumn, but both of them are full of jokes and have no interaction with the audience. In fact, backwater appeared for the first time as an interactive link, which should be the interactive joke in 98 Xu Genxiu.