One of the staples of a San Francisco 49ers defense that’s ranked No. 2 in the NFL in points allowed is the pass-rushing twist with outside linebacker Aldon Smith and defensive lineman Justin Smith against left guards and tackles.

On the call, Justin Smith lines up between the guard and tackle and tries to occupy both by crashing into the tackle and pulling the guard with him, while Aldon Smith loops behind and bursts through the gap at guard. Both Smiths were voted to this year’s Pro Bowl, and Aldon Smith finished second in the NFL in sacks with 191/2.

“The key I’d say is for the guard to not let Justin get his arms around you and engage you,” Packers left guard T.J. Lang said, “because that’s when it’s most effective, when he’s able to lock up on that guard and give him a little tug to the outside, making room for (Aldon Smith) coming around. My job is to make sure that doesn’t happen.

“And we talked about for the tackle, his job is as soon as he sees Aldon going inside, come down with the same force and hit (Justin Smith) and try to knock your guard (i.e., Lang) off to the inside. Me and Marshall (Newhouse) defended it pretty well the first time we played them, there was probably one or two they kind of leaked on us.”

Just after the snap, it’s difficult for Lang to know whether Justin Smith is doing a conventional rush or the stunt, so Lang relies on Newhouse to see Aldon Smith looping behind. Newhouse then bumps Lang as a physical signal that the twist is on.

“That (bump) tells me I have to do anything I can to unlock from (Justin Smith) and try to get to the inside and shut it off,” Lang said. “I think Marshall, that’s one of the better things he does, recognizing those, making sure he’s coming down not just putting his hands on guys but throwing a shoulder in there with some force and knocking me off.”

Two NFL scouts this week said Justin Smith is an expert at holding the left guard and not getting called for a penalty. One reason Justin Smith’s torn left triceps could be a major factor in the game is that’s the arm he uses to pull the left guard with him into the left tackle.

Justin Smith sustained the injury four weeks ago, and this will be his first game since. He’ll be wearing a brace to prevent his arm from flexing, which could limit his ability to use his hand and arm.

“Sometimes he does (hold) and sometimes he doesn’t,” Lang said. “But I don’t think I’ve ever seen one penalty called on him when he does that. That’s just the way he plays. He likes to get under you, he likes to get his arm around you, kind of hug you up, whether it’s on that twist or whether it’s on a bull rush, it’s very difficult to defend if you don’t see it coming. That’s why we practiced it a lot this week. It’s something you’re never going to see called, you just have to find a way to get off him.”

Said offensive coordinator Tom Clements: “It’s not holding if it doesn’t get called.”