Barry wrote earlier about the safety Niners coach Jim Harbaugh declined in last night's game. The safety would have made the score 15-6 rather than 13-6, and would have pleased those who wagered on the Niners (who were -7 at most books) and vexed those who wagered on the Seahawks. Barry also noted (correctly) that the call against the Seahawks was a terrible one.
A chop block is a thigh-or-lower block on a defender who is already engaged with another offensive lineman. (Chop blocks and cut blocks are not the same thing. A cut block, which is legal, is a below-the-waist block involving only one defender.) Guard Pat McQuistan, as you can see in the video above, blocks Aldon Smith at the waist. That's not a foul, just good offensive line play. Only if McQuistan had hit Smith in the thigh or lower—even if Smith was not physically engaged with tackle Breno Giacomini but preparing to rush him—would the flag have been justified. Blockers never can go high-low on a defender on pass or kicking plays.
But not all chop blocks are illegal. On running plays, offensive linemen can go high-low. The NFL rulebook gives three such situations. Two offensive linemen who had lined up next to each other on the line of scrimmage may chop a player whenever they'd like. Two offensive linemen who had lined up with one or more linemen between them—e.g., a center and a tackle—can chop a player, too, so long as the play is moving toward the block. For instance, a right tackle and a center can chop a defensive end on a run to the right, but not on a run to the left. Backs can chop in concert with linemen, too, so long as the block takes place outside the tackle box.
The NFL's reasoning behind all these proscriptions is that defensive linemen are much more likely to be injured on low blocks when they can't see them coming. On a run-stopping rush, the thinking goes, defenders are eating up space, preparing to make tackles, and following the flow of the play: they're more likely to be aware of what awaits them, especially if it comes from two adjacent linemen. In pass-rushing situations, there's no flow to the play, and defenders are running hard at one (mostly stationary) target. A chop block surely would catch a pass rusher off-guard.
To an outside observer, the rulebook's distinctions are unconvincing: Why is the Texans' admired zone-blocking scheme any safer for opponents than what the Seahawks stood accused of doing? A player's lower extremities are endangered in either case. But this is often the case with injury-related rules in the NFL: The league tries to impose bright lines, and human physiology rejects them.