Exactly what Bowwild said. I switched to 3 under about a month ago and was not using a second nock on my string. About a week ago I went to a second nock. This has helped keep my tab from coming into contact with the  arrow nock. I can say that I did notice a difference right away. More consistent arrow flight from shot to shot. May not matter to others but it did make a difference for me. I just use the brass nocks.