What was the original question you intended to ask? AFAIK, the momentum exception applies when there is a change of possession, and the player who possesses the ball steps into his own end zone after having possessed the ball between his own 5 yard line and goal line. Usually, when the ball enters the end zone, it becomes dead there. However, because the ball was possessed in the field of play, and the player in possession of the ball had his momentum carry him into the end zone, he is allowed to keep possession of the ball at the yard line where possession was initially acquired (unless he subsequently fumbles or fouls). This is what is known as the momentum exception.