This maybe better then me trying to explain it:
From Foxfire website:
Note how the feathers are positioned with our 60/120 4 fletch in relationship to the shelf. There no longer are any feathers striking the shelf, while the arrow goes through paradox. Therefore, one can use a lower nocking point. For many years, I used the common 90 degree four fletch and had excellent results. My nock point usually was quite high, like 3/4 “ above square, or more. When, my good friend Roger Rothhaar told me about the 60/120 degree four fletch many years ago, I had to try it. Wow, was I surprised! I was able to lower my nocking point over 1/4 “, which gave me more trajectory without having to raise my point of aim. The best part though, was improved flight.