http://www.espn.com/blog/nflnation/post/_/id/262186/the-nfl-catch-rule-is-dumb-but-the-jesse-james-call-was-right
"A player who goes to the ground in the process of attempting to secure possession of a loose ball (with or without contact by an opponent) must maintain control of the ball until after his initial contact with the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, there is no possession.
"A player is considered to be going to the ground if he does not remain upright long enough to demonstrate that he is clearly a runner."