steelers@ravens winning touchdown

[newmdref]

I would like to actually ask a rule question as it relates to this situation. (I am a new official so go easy on me) I understand its a touchdown by rule when the receiver is in possession of the football, with both feet down in the field of play and any portion of the ball intersects the EZ line. My question is at what point does the receiver establish possession?? It is my understanding (limited understanding) that the receiver must make a move with the football and maintain possession of the ball through that movement in order for possession to be established. When I watch this play on tape the receiver's first movement(tucking) of the ball takes the ball out of the endzone, if it was ever in, which establishes possession. So...in this situation was possession established a the second it touched his hands?? or after that??. A "What if" situation I have thought of, as it relates to this play, is: the receiver catches the ball just as he did in the game but is hit from behind almost immediately after his hands touch the ball (with both feet in bounds and the ball touching the EZ line) and losses possession of the ball while attemping to tuck it. Is it still a TD or incomplete pass?? Please Advise.