As the Bills were driving down the field to tie the game there was a catch for a first down by Nelson who, after hitting the ground, coughed up the ball. The refs ruled it a catch and a first down and the Bills quickly went to the line of scrimmage, snapped the ball and grounded it to stop the clock. Then the refs said the play before, the Nelson catch, was being reviewed. How do they go back and review a play after another play has taken place?