If a fumble or backward pass by any player goes out of bounds, the game clock starts when an official places the ball at the inbounds line, and the Referee signals that the ball is ready for play.
While Flaccos fumble was accidental, the idea of the rule is to protect from players intentionally fumbling out of bounds to stop the clock. For example, imagine Flacco knew the sack was coming and realized he wouldnt be able to pass downfield, so he rolled the ball out of bounds.
Apparently, the Ravens didnt know the rule, though. The team attempted to huddle while the clock ticked down to zero and ended the game. Now they know