When Allen first covered the ball in the last two minutes to pin the Bills at their own 1, I thought that the officials were going to call it a touchback, and place the ball at the 20. Why? Because Allen was clearly in the endzone before the touched the ball (see the first pic).
But I was wrong, because he "established his feet in the field of play" before he touched the ball. The rule on this one is sometimes mis-applied and mis-understood. It basically allows for a players to step into the endzone, and step back out - but he has to get both feet down outside of the endzone, and make a move; and no part of his body can be in the endzone.
He did just that, as you can see in the second shot - and the official happens to be right there watching the play, and its his responsibility to ensure that Allen did establish himself before touching the ball.
And as you can see in shot 3, it is clear that Allen is the first one to touch it. So kudos to the ofificals for getting this one right - it would have been easier to simply say it was a touchback.