It's used as more of a method to trick you into telling the truth than it is to detect lies. I don't think it's very easy to beat unless you teach yourself how to regulate your heartbeat.
You don't have to control your hartbeat, you have to overreact to control questions (performing heavy mental calculations combined with holding your breath 2 seconds will do the trick!), so your (normal) response to the important questions will be less severe in comparison.
If your response to control question is stronger than your response to relevant questions, you pass the test. It that simple.
There's a sensor that's strapped around your chest. If your breathing is irregular, they'll detect it. Also, doing anything mentally draining while someone is asking you a question and answering it correctly is not easy. At some point, you're going to have to focus on the question. You'd have to be able to consistently deceive them for every question and keep at it for several hours.
They don't necessarily tell you which questions are the control and they'll slip in questions that you'll have to think about and answer honestly. They'll catch your baseline with those. At times, they'll intentionally try to agitate you by accusing you of lying. It's difficult to keep cool and it will throw you off balance. You'll forget that you were trying to trick them.
The government knows about all of these stupid little tricks. People used to put pins in their shoe or contract their anal sphincter muscle to generate a response. Examiners now have sensors under your feet and ass. These tests aren't designed to simply detect that you're lying. They want to push your buttons and make you crack.
You can be stubborn about it and lie and insist that they've got it wrong, but your reading will show that you're holding back on them. If they sense that you're trying to deceive them, they'll fail you.
Cheers