https://www.cougarboard.com/board/message.html?id=24736917
But then I went to the rulebook. I only looked in the NFL rulebook, but there actually wasn't any established yardage threshold for momentum. It simply says if momentum carries the defender into the endzone, the ball is brought back to the spot where possession was gained. It would only be a safety if the defender made an intentional and purposeful move towards the end zone.