OK, I would have gotten this wrong... and I don't understand the rationale.
8-5-2-a Initial Impetus
ARTICLE 2. a. The impetus imparted by a player who kicks, passes, snaps or fumbles the ball shall be considered responsible for the ball’s progress in any direction even though its course is deflected or reversed after striking the ground or after touching an official or a player of either team.
2-16-1-a Kicking the Ball; Legal and Illegal Kicks
ARTICLE 1. a. Kicking the ball is intentionally striking the ball with the knee, lower leg or foot.
b. A legal kick is a punt, drop kick or place kick made according to the rules by a player of Team A before a change of team possession. Kicking the ball in any other manner is illegal (A.R. 6-1-2-I).
c. Any free kick or scrimmage kick continues to be a kick until it is caught or recovered by a player or becomes dead.
I'm not understanding that since a kick is a kick is a kick, why that isn't considered new impetus, as I understand it from 8-5-2-a?
Impetus is what causes the ball to travel from the
field of play into the end zone, even if that impetus is imparted on the ball while it is in the end zone. This is easy enough to envision when the ball is fumbled, passed, kicked or batted in the field of play, then travels across the goal line where it becomes dead. However, it may be a little harder to envision a ball getting fumbled, passed, kicked, or batted while in the end zone, and then it travels into the field of play, then returns over the goal line into the end zone, where it becomes dead. But that is what this scenario offers. The ball was already in the end zone from the batting by Team B in the field of play. Had the ball stayed behind the goal line and become dead, even after A4's kick, the impetus would have been from the batting, and the result would have been a touchback.
Impetus on the ball was still with the batting until the ball traveled into the field of play. Then, when it returned into the end zone, the kicking of the ball is what caused it to travel from the field of play, back into the end zone where it became dead. Thus, the impetus was from the kick.
To reinforce this concept, if a punter receives the snap in the end zone, at that moment, impetus is with the snap. Even when he kicks the ball, impetus is with the snap, until we know what happens with the kicked ball. If it goes directly OB in the end zone, the result is a safety, but not because of the kick, itself. It is because Team A put the ball into the end zone from the field of play with the snap, and then the ball became dead there. Impetus remained with the snap.
By the same token, when Team A punts the ball, and a Team B players catches the ball in the end zone, then, while still in the end zone, throws a backward pass out of bounds behind the goal line, that's just a touchback, because Team A is responsible for the ball being behind B's goal line (impetus is with the kick), where it became dead.