Completed Pass becomes Incomplete

I’ve seen replays of the “touchdown” the Lions scored against the Bears in the last seconds of the game that was, after review, called an incomplete pass. I heard some explanations, but none makes sense to me.

The receiver clearly caught the ball. One explanation I heard was that since he caught the ball in one hand when he went down, he had to put the other hand on it. In the first place, I believe he caught it with both hands, but I may be wrong on that. He clearly had possession when on the ground, but placed the ball on the ground before getting up. Is that the key? Is the answer that when you dive to the ground on your own volition (not tackled or pushed), you must regain standing position in possession of the ball? If so, that rule sounds inane to me. What purpose does it serve?

Moving to the Game Room from GQ.

May as well lock it, as the subject is already being discussed in the Week 1 NFL thread and the “Lions got screwed” thread.