The NFL sought to clarify what a completed catch is, re-wording the rule after a controversial incident in a play-off match.
Dallas Cowboys wide receiver Dez Bryant was denied what appeared to be a catch in the final five minutes of their 26-21 loss to the Green Bay Packers, who advanced to the NFC Championship game.
A statement from the league's vice-president of officiating Dean Blandino read: "The committee looked at the language and made several changes.
"In order to complete a catch, the receiver has to have control, both feet on the ground, and he has to have it after that long enough to clearly establish himself as a runner.
"This would fall directly in line with our defenseless player rule, where we say a receiver is protected until he can clearly establish himself as a runner. And what does that mean? That means he has the ability to ward off and protect himself from the impending contact."
Blandino added: "Well if he can clearly establish himself as a runner, then he's not going to the ground to make the catch.
"If he hasn't clearly established himself as a runner prior to going to the ground, then he has to hold onto the ball until after his initial contact with the ground."
The play - which came with the Cowboys trying to convert a fourth-and-two - saw Bryant catch the ball, but in the process of landing, the ball fumbled in his grasp.
The on-field call was a completion, and the Cowboys would have had a first-and-goal situation within the one-yard mark, with a touchdown putting them in front.
But a Packers challenge saw the call overturned.