Arizona Cardinals Defensive Coordinator Ray Horton doesn't appear to particularly enjoy the 49ers use of a wily move on 3rd and 4th down situations of short yardage. Basically, the 49ers first send a receiver in motion once set at the line. When the receiver goes to line up on the line of scrimmage, the tight end goes to take a step back to avoid an illegal formation penalty.
↵However, the step back is generally hard and abrupt - or hard and abrupt enough to look like a false start. Twice, once against the Seahawks and once against the Giants, an opposing defensive lineman has jumped across the line thinking it was a false start and subsequently has been called for encroachment. First down Niners.
↵However, the 49ers were also called for false start while executing this play against Washington, so there is a measure of vagueness to the rule. It sort of straddles the line of legal and illegal, and Horton had this to say:
↵↵↵"It's a cute play, and the league has said it's a cute play also, a legal cute play. So kudos to them for having cute plays in."
↵
While the NFL rulebook explicitly states that "No [offensive player] may...move abruptly...in such manner as to lead defense to believe snap has started," the way the 49ers execute the motion leaves some room for subjectivity. Either way, expect Jim Harbaugh to continue to use it as long as it's working!