Sima Yi stood behind Cao Cao and beside Xu Shu, observing Xu Shu's performance. Xu Shu remained silent, and Sima Yi wanted to express his views. Xu Shu shook his head, but Sima Yi did not speak.
The main reason is that Sima Yi assisted Cao Pi at that time, when Cao Pi and Cao Zhi had fierce competition. Sima Yi kept a low profile and didn't want to show his face too much, so he was always cautious for fear of causing Cao Cao's disgust. Nevertheless, before he died, Cao Cao said that Sima Yi's wolf looked after kites and could not give military power.
Compared with Sima Yi, Yang Xiu showed more wisdom, while Yang Xiu helped Cao Zhi too much and was finally killed.
It was not until Cao Cao's death that Sima Yi assisted Cao Pi and really stepped onto the political stage.