You're not allowed to hit the quarterback from the knee down. It's as simple as that. In fact, you're supposed to "make every effort to avoid such contact." That's the rule.
http://sports.espn.go.com/nfl/news/story?id=2389062
Safety was also a big winner during Wednesday's meeting. Here are the rules that passed involving safety.
• A rushing defensive player won't be allowed to forcibly hit a quarterback below the knees. He has to make every effort to avoid such a low hit. Palmer, Griese and Roethlisberger suffered knee injuries on low hits but those three plays were considered legal by the committee because they involved defensive rushers coming off blocks. Several other plays such as the old Rodney Harrison hit on Trent Green when he was with the Rams along with a Jared Allen low hit on Kerry Collins would be subject to a 15-yard penalty. That proposal passed, 25-7.