Wait why did he say to tie his mouth? I don't understand can someone explain please

I’m guessing the suggestion thing they were talking about makes the person who it has been used on kill themselves if they reveal any information. Since suggestion is used to controlling the people from what I understand. So I’m guessing he did that to prevent him from dying. But I could be wrong I didn’t read the novel loll

He's probably trying to kill himself by biting off his tongue, so that's why they tied it up