Out of bounds play in Lions/Tampa game

[tommyk]

My friends and I are trying to figure out a play near the end of the Lions Tampa Bay game this sunday. The Lions completed a pass to Williams. Williams then stepped out of bounds. This was in the last five minutes of the game. For some reason, the clock didn't stop. Someone said it was because he was not moving forward when he stepped out of bounds. I've never heard of such a rule. Does anyone know what the rule is in this situation.

Also, is the official NFL rulebook online? I know NFL.com has a kind of summary, but I can't find the official rulebook.

Thanks.