Being a retired Law Enforcement professional my opinion of the Poly is.....
It is just a method of interrogation, if the operator is a skilled interrogator then he tricks the subject into telling the truth or crossing up his lies.
The scary part about the test is when the operator believes him or herself to be skilled. In the criminal world for the most part the subject is guilty and they just need more evidence against him.  But, for all of us normal people out here just trying to get a job it is ridiculous.  I know this because I tested three different departments before I finally passed a poly. I know I wasn't lying.  I also went to the poly school of deception a few years later.
It is the same as if you are tied up in a room being scared out of your wits and light torture to break the truth out of you.  Very few of the questions asked of you are true yes and no type questions. Such as prior to the test they will ask if you have ever stolen anything.  You say some candy bars when you were 10 years old they clarify and ask if there was anything when you were older you say no.
Then as you take the test and he asks for that yes or no answer your mind wanders and you wonder if you should mention when you got home from the store and realized you hadn't paid for a pack of gum, or that pen you took from work even though you used it for work related things at home, or or or or .....  He sees you thinking about these things and he jumps on it not knowing that you are still thinking about that stupid stealing question since he has moved on to others he then turns off the machine so he can talk to you about it. And this is when he determines that you are lying about your drug usage his current question  and he never believes you really didn't do any drugs while he  keeps pushing the drug question after turning off the machine and you keep saying no, since he cant get you to admit to a little of it he decides you are lying..
So just relax say "yes" or "no"... don' tthink or try to read more into the question... also note if you relax too much they will think you are a liar because they can't catch you.