What is Zhou Yu's clever calculation of borrowing the arrow from the straw boat?

Count the weather: It's a good idea to borrow an arrow from the east wind. Method: It's a good idea to borrow an arrow from a straw boat. Lu Su: I can't summarize this. Anyway, every time Zhuge Liang can be sure that Lu Su will come to him, and he can also figure out that it is a good idea for Zhuge Liang to send him by Zhou Yu. Zhou Yu: Sanyu is a good idea. Cao Cao defeated Hua Rong, I hope it will help you.