Only once the forward motion of the arm is completed, and the ball tucked into the quarterback's body, would a subsequent loss of possession be considered a fumble.
The tuck rule was called in Week 2 of an NFL regular season matchup on September 23, 2001, between the New England Patriots and the New York Jets.
"[4] The tuck rule resulted in a controversial finish to an AFC divisional playoff game on January 19, 2002, between the New England Patriots and the Oakland Raiders.
After instant replay, referee Walt Coleman reversed this call, declared the play an incomplete forward pass, and gave possession back to New England.
[5] In later interviews, Coleman stated that it was his explanation, not the reversal, that was in error; the ball was moving backwards when it was lost, but the tuck rule applied.
The tuck rule was enforced in a regular season game on October 9, 2005, between the Washington Redskins and the Denver Broncos.
[1] The tuck rule was also enforced in an NFL playoff game on January 9, 2011, between the Kansas City Chiefs and the Baltimore Ravens.
After this game, Mike Pereira, the former NFL vice-president for officiating, stated that he was no longer in support of the tuck rule.