I'm not really sure exactly, but I believe its a stop to get eligible for college. I'm not sure how they decide, some players defer enrollment until January, while working on test scores on their own, so I guess its more for grade issues than test scores. Also, I think its for people that might be closer academically than some, b/c apparently you have four years of eligibility out of prep school as opposed to going to a JUCO and it counting towards eligibility. The best example(I think) is Larry Fitzgerald a few years ago. He went pro after his true sophomore season, but he appealed to go early b/c he spent one year at a prep school. I'm sure somebody else has a better explanation, but thats my understanding of it.