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)?
It has to do with how they spot the ball for the next play. If any part of the ball is in the end zone (and the lines are part of the end zone), then they would be spotting the ball in the end zone for the next play. Ridiculous, no? Ditto for safeties -- if any part of the ball remains in the end zone, it's a safety.