The whole ball or just part of the ball? You see where I'm going here, right?
Yeah, interpretation, interpretation, interpretation. Entire ball. It's definitive. If it's good enough for the NHL (entire puck has to cross the red line for a goal), it should be good enough for the NFL.
What is the origin of this plane rule? Pile-up on the goal-line with a QB/RB/FB extending the ball (i.e., no one touching the ground)? Minimizing injuries (e.g., Marcus Allen flying over a pile into the endzone)?