The skin punch is when they take a circle about the size of a dime from (in my case) 3 areas - hip, leg & ankle. It goes about 1/16" deep, and is just a band-aid to heal.
No shower or bathtub (just sponge bathe, away from areas) for 5-7 days, and about 2 weeks more to heal completely.
Non invasive, can't feel it cause they numb the area beforehand.
Then it is sent off to the lab to be stained and read under an electron microscope to tell if the small fiber damage is there.
Takes special labs, stains, and trained (lab techs & docs) interpretation to do it correctly. I had mine done at Hopkins.