The defensive player who is in pursuit of a quarterback has to make a reasonable effort to avoid hitting the passer once he releases the ball. A late hit on the quarterback after he has thrown a pass is often called "roughing the passer," and it results in a 15-yard penalty and an automatic first down. It is a judgment call by the referee, who is the official positioned in the offensive backfield, as to whether the hit could have been avoided.
Hitting in the Head
Hitting a quarterback in the head is always a foul, even if he is in possession of the ball. Defensive players must get to the quarterback and hit him below the helmet to avoid penalty and record a sack. A blow to the head can be as simple as a slap and as violent as full helmet-to-helmet contact, but both are penalized the same--15 yards.
Hitting Too Low
One of the NFL's newer rules is often called "The Brady Rule" as it was added after a season-ending injury to New England Patriots quarterback Tom Brady in 2008. This rules states that a defensive player who hits a quarterback below the knees when one or two feet are on the ground will be penalized for a personal foul and a 15-yard penalty. The hit must be above the knee to be legal. This penalty is often called on pass rushers who are blocked to the ground, but they continue to scramble toward quarterback and end up hitting him below the knee, exposing him to serious injury.
Grasp and Control
The NFL has had a rule on the books for many years that calls for the referee to blow the play dead if a defensive player has the quarterback in his grasp. The term "grasp and control" is used to define this rule. It is up to the judgment of the official on the field, who must decide if the quarterback's ability to make a play has been stopped because he is under the control of a defender who has a hold on him. If he feels there is control, he will blow the whistle and the play is ruled a sack at that spot.
The quarterback is not allowed to intentionally throw the ball away to avoid a sack. If he is outside the offensive tackle on either side, he can throw the ball beyond the line of scrimmage to avoid a sack. If the quarterback is in the pocket formed by his linemen and he throws the ball to an area where there are no receivers, he is guilty of intentional grounding. The penalty moves the ball to the spot of the foul and comes with a loss of down.