Aren't they supposed to work by measuring increased heart beat in someone likely not to be telling the truth?
Increased heart rate, blood pressure, respiration rate, pupil dilation, eye movement, skin conductivity, all sort of things. However, the results depend on the reactions to "control questions" and previous training, psychological states and conditioning and self-belief even psychopathy can confuse the results. As the test itself is an abnormal condition then the above physical changes will be elevated to start with. Self-inflicted minor pains or muscle cramping during the "control questions" reduce the difference between the "control" and "actual" questions.
So if these things are "reliable" then why are they not in public use so we can dispense with trials, juries, investigations, appeals and just go from "rounding up the usual suspects", one simple test and then straight to the death penalty? If they boast 96% accuracy then that is better than any justice system. In fact the machine could just "fry" the victim at the end of the "reliable" test.
Be popular in Texas.