Explain this penalty to me.

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.

That's one scenario that comes to mind.

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: