Their professional analysis involves operating a machine. They don't know if you're lying or not. Part of the whole charade is convincing the person taking the test (most commonly Law Enforcement Officer candidates) that it works, and as a highly trained professional and expert polygraph examiner, there is no way you can lie and get away with it.

The most effective use of a polygraph is deception: convincing the person being tested the machine actually can tell that you are lying and therefore disclosing the truth. e.g. "I'm getting an abnormal reading on that drug use question, I'll ask you one last time...Have you or have you not used illegal drugs in your lifetime?" "Okay, well there was this one time...." Bingo, disqualified.

Not taking any sides here, just pointing out polygraphs are bs.