From Dave Naylor`s TSN column:
There was some concern raised over the weekend about plays where defensive backs appeared to be defending receivers without ever turning to look at the ball, including on the play where Montreal's Chip Cox broke up a two-point convert attempt throw to receiver Aaron Kelly that would have tied the game for Hamilton. In the past, defenders who do not turn to look at the ball before it arrives have been flagged for faceguarding. However, CFL director of officiating Tom Higgins said the interpretation of that foul has evolved in recent years so that defenders are no longer required to turn and look at the ball before making a play on a receiver as the ball arrives.
"The rule has changed over the course of a decade," said Higgins. "Defensive backs need not be turning back to play the football to cover a receiver. Defensive backs are now taught and coached to read the eyes of the receiver and that's a good defensive play. You don't have to look at the ball. You're playing the ball by looking at the receiver and trying to time when the ball is going to come. The defensive back has only one shot to shoot his hands because if he does it too early they are going to lose speed. You can stay on the receiver with your body and your hands and read his eyes and his body without turning your head."
"Faceguarding is a deliberate attempt to put your hands in the eyesight of the receiver for an extended period of time."
I always thought the defender had to turn his head back, but I much prefer this new? interpretation.