The "real" TLDR (and I am not kidding here!) is that you clinch your anus during every answer. It does something to your heartrate, blood pressure, etc. that makes lies indistinguishable from the truth. Also polygraphers know this and will call you out on it if they think you are doing it. It makes all of your answers register the same and they assume that you will lie on at least one as a baseline (usually something like "have you ever told a lie in your life" or something similar).