It's the ball. It depends on where the ball is possessed. If it touches the goal line while still loose, it's a touchback. If the player on the goal line possesses the ball inches outside the goal line, even if he takes a step back, he is a runner.
I had not seen the scenario where a player on the goal line possesses the ball barely outside the endzone in years, and I had this call (or no call, as were the cases) twice this year, in successive weeks.
This post was edited on 10/30 2:38 AM by White hat