The fifth provision of Rule 12, Section 2, Article 12 (roughing the passer) says that: "A rushing defender is prohibited from forcibly hitting in the knee area or below a passer who has one or both feet on the ground, even if the initial contact is above the knee. It is not a foul if the defender is blocked (or fouled) into the passer and has no opportunity to avoid him."
linkIf the defender is on the ground he can still try to arm/hand tackle the QB, but he can't "lunge" or hit his knees (or lower) with any other part of his body.
So now we have to sit through replays to see if a players knee was down when he sacked the QB???
It is an addendum to the roughing the passer rule and is not subject to review.
What I don't like is that it is very subjective on the part of the refs, they already have to much influence on game outcomes, this just puts more of the game on the refs instead of the players (you know, the guys we actually pay to see)