If you've got a tackle uncovered (eligible) and he doesn't report, that's an illegal formation. Shift to cover him and you're legal, but you didn't start out legal. Flag.
Thats a good explanation, and i'm sure its probably true, but man it sure seems like every week during a game somewhere, that they whip out some obscure or new rule that makes it seem like they make it up as they go along. :hihi: