Tucson News reported that there are not enough CBP agents to handle all of the Trusted Traveler Program applications that require face-to-face interviews. It works by using sensors "to screen passengers for unusual physiological responses to questioning — which can indicate a subject is lying," according to CNN.
It's not what you answer, but how you answer. Are you upset or fidgeting? CNN reported that it "uses three sensors to assess physiological responses: a microphone, which monitors vocal quality, pitch and frequency; an infrared camera, which looks at pupil dilation and where the eyes focus; and a high-definition camera recording facial expressions."