The idea isn't that the machine is actually detecting anything here, it's that the general atmosphere of the "test" and a sufficiently convincing performance of

The idea isn't that the machine is actually detecting anything here, it's that the general atmosphere of the "test" and a sufficiently convincing performance of "the machine says you're not being truthful" will get people to volunteer information that they would otherwise have kept to themselves. It's a variation on an old technique where an interrogator will claim to have incriminating evidence or eyewitness testimony that they actually don't.