[steveegg]

Let's say a receiver jumps in the air, catches the ball and is immediately hit by a defender, lands on his back and the ball pops out. Is that a catch or not?

No catch because the player has to maintain control of the ball through contact with the ground. Indeed, that rule was changed a couple years ago to make that specific (before, it had been a judgement call of whether the contact with the defender-incomplete or contact with the ground after contact with the defender-typically complete had caused the ball to pop out).

[JeffAtlanta]

No catch because the player has to maintain control of the ball through contact with the ground.

That is the end result, but the lack of a football move before losing control of the ball is the reason it is an incompletion.

The logic is that if a reciever catches a ball cleanly and it driven to the ground by a defender, then the play should be over the instant the reciever's back touches the ground.

The additional requirement of "football move" is what makes the above scenario an incomplete pass.