Unfortunate, but poor language, or at least poor language coordination.
8-7-1 states that impetus is the action that forces the ball on, above, or behind a goal line. Impetus is irrelevant if the ball doesn’t become dead on, above, or behind a goal line. Therefore, only the action that puts the ball on, above or behind the goal line, where it becomes dead, is the impetus that caused it to be there. Impetus can’t be assigned until the ball becomes dead on/above/behind the goal line.
If a free kicked ball gets batted at the B-5, And the ball travels into B’s end zone, where it becomes dead, it ain’t the kick that put it there - it is the batting action. The batting is THE impetus that caused the ball to be there.
If B intercepts the ball in B’s end zone, then B fumbles, and the ball travels into the field of play, where it deflects off of another player and travels back across B’s goal line, and rolls out of bounds, it wasn’t A’s pass that caused it to become dead behind B’s goal line - it was B’s fumble, which was THE impetus.
There is only one impetus, and that is the action that puts the ball on/above/behind the goal line, where it then becomes dead. By its own definition, impetus can’t be changed. If an action on the ball is not the one that forced the ball on/above/beyond a goal line (where it then becomes dead) by definition, it can’t be impetus.