With regards to a QB;
- No defensive player may run into a passer of a legal forward pass after the ball has left his hand (15 yards). The Referee must determine whether opponent had a reasonable chance to stop his momentum during an attempt to block the pass or tackle the passer while he still had the ball.
- No defensive player who has an unrestricted path to the quarterback may hit him flagrantly in the area of the knee(s) or below when approaching in any direction.
- Officials are to blow the play dead as soon as the quarterback is clearly in the grasp and control of any tackler, and his safety is in jeopardy.
Essentially, the only ''target area'' of the QB is his waist and torso. Even going for the thigh area can get you into trouble. If tackling the QB from his blindside - his back - a player must also do his best to ensure his helmet does not come into contact with the QB's head/neck. A tackler is also not allowed to use his forearm to charge a QB. You can't touch them and they must blame Tom Brady and Peyton Manning for that nonsense. Alot of this is also really tough on tacklers as most pass rushers, wether linebackers or defensive ends are huge and cannot stop on a dime to readjust themselves if a QB moves. I've even seen players penalised for falling into a QB's knee after they have been pushed in the back by an offensive linemen.
And yeah, they've gotten very tough on helmet-to-helmet hits.