Next the Cao Cao humiliation conga. Cao Cao is retreating and trying to escape, and when he comes to a fork in the road, he sees smoke down one path. He decides that the enemy forces will not let the smoke of their campfires give away their position, and that this must be a decoy, and that they knew that Cao Cao knew that they would not give away their position so the clear path must be an ambush.
So, Cao Cao takes the path with smoke, and gets ambushed, because they were there. They knew that Cao Cao would think that they would not give away their real position so they set the ambush on the path with smoke since Cao Cao will believe he is being tricked when it's actually the truth.