You clench you butt cheeks when they do the calibration when you want something to register as a lie. So you tell a truth and then clench your butt cheeks for calibrating a lie. Thus they get a strong response on a truth/lie and a less strong response on a lie. The other trick is to recall a traumatic event.
https://www.mirror.co.uk/news/weird-news/how-cheat-lie-detec...