Ex agency. Have taken many polys. Do they work perfectly - heck no. Are they an important tool that should be used - heck yes.
And being nervous isn’t much of a factor - that’s taken to account in your baseline.
The operator “calibrates” the polygraph by asking a false question and you are supposed to answer in the affirmative. Like is today Wednesday(actually it is Monday). You answer yes and the needles are supposed to bounce around.