I have never been poly'ed, but my sister and my stepdad both were.
My dad, his was for suspected embezzlement. He was not charged, just a suspect and was later cleared. But he said that they took initial heart rate and BP readings when he got there and then he just talked to the interviewer for a little while. THen they rechecked his BP and heart rate and all that and made him remove his shoes, roll up his sleeves, so on. THey hooked him up, to a BP cuff and a heart monitor thingie and then started asking him questions. He said he didn't feel nervous at all, because he wasn't guilty. They went through questions about his age, race, gender, parents, address, things with known answers, then they moved onto questions requiring a yes/no answer only, and then the actual questioning.
My dad failed, but not because the test showed deception. He failed because the test showed no difference in questions where he was being truthful and control questions where he was told to lie. The only thing the interviewer told him was "Looks good, we're done here." He got his results about 3 weeks later.
My sister was a nervous wreck when she took hers and she didn't tell me much about it, except that she got done with the test and broke down. She told the examiner that she was afraid she was going to be arrested because of how nervous she was.
The examiner said that if she was as nervous all the way through as she was when they started, she should be okay. He said nothing about her results.
Hers were inconclusive, because the test showed that she was lying on almost every question, including known answers, such as her age, DOB, address, children's names.
But might I add, she also has a heart condition and a thyroid issue, which they did tell her later might have added to the difficulty in reading the results. She never actually got her results sent to her, the police brought them to her about a week later when they came to ask her some more questions.
ETA: My dad's was done a long time ago, like the early to mid 80's, so it was much less sophisticated.
My sister's was more recent, about three years ago and she said that they used a computer to read the results and the person administering it told her that he would be doing the evaluation, but due to the seriousness of the accusations, the results would be interpreted to police before they were given to her.