Fields has a special gene, like Kevin Love. He just simply knows where the ball is going to go once it hits off the rim.
Think about it, when you play pick up even, there's always some guy who isn't huge, isn't all that athletic, isn't all that good...but he always seems to be in position to get the rebound, where it falls right into his lap.
He's a Stanford educated player who can tell where the ball is going when it is shot and is able to get himself into position. You can't teach it, it's just natural smarts and ability to be at the right spot at the right time.