So I've read more now..finning is because it was struck confined by the collar. (It was struck within the die neck and collar) .. I have to admit I get so confused sometimes. (Splitting planchet errors either before or after is where I have to read additionally I guess. Now that I've been led into learning more
All modern coins are struck in a collar. When the striking pressure is too high, metal will squeeze out in the minute gap between the collar and anvil die. This thin metal is called a fin.