Earlier quoted context omitted.
They're based on the idea that if you're made to tell a lie, your body will react in some way (sweating, faster heartbeat etc) compared to a baseline measurement. There is a whole field of psychology around lying and additionally agents are ostensibly trained to detect visual cues that someone is telling a lie. The premise isn't exactly junk - hardware lie detectors are basically trying to identify a fight or flight…
It's borderline useless. And can do lots of harm if assumed that it's useful.
I wouldn't disagree with that. And a big problem is that innocent people tend to produce stress reactions when interrogated, leading to false positives. It would be appropriate to call a polygraph a "fear" detector, since that's basically what it's measuring. But still, the "science" isn't entirely bunk: some people do panic when they lie - though I read an anecdote that said this is a weirdly American trait (Europeans can cheat the polygraph more easily) - but it's virtually impossible to sift out the true positives from the noise.