brett favre

[53Fan]

I agree that there should be some rule changes which brings me to this question that maybe someone can help me with. I thought after Brady got hit low and was injured a year or two ago, the rules were changed so that it was illegal to hit a QB around the knee area. In fact I've seen it enforced even when it was impossible for the defender to stop. Favre was clearly hit low on the sack he appeared to get hurt on and the defender was clearly not trying to hold up. There was no flag, what am I missing?