Lie Detector

So my parents got this gift for Christmas, a lie detector that shocks you if you lie. I was, of course, skeptical and really curious to find out exactly how it can tell if you are supposedly lieing or not. This is a simple machine so I really doubt that it can check heart rate or moisture of your skin and certainly not pupil dilation. The only thing I could figure was that maybe it senses the pressure of your hand pushing down on it. Theoretically if you know something will shock you when you lie you will try to avoid being shocked and so if you lie you will try to pull away from the thing that will shock you. So if you lie you should release pressure on the device. If it senses that then it might think you are lieing and shock you. That's my theory. So I tested it out. I managed to shock my brother and my dad by having them release and apply pressure a certain way. I can't think of any other factors so I believe that that is how it works.