Lemmy give an example (simplified and exaggerated for effect)
Person 1: “I support BLM.” Person 2: “You are a racist who wants to take the lives of all non-black people, and if anyone defends person 1, then they are racists as well.”
Nobody wants to be called a racist, so people will get very angry and start arguing that they are not racists, and the troll will get what they wanted.