Stoppage Time Bleg

| Sun Jul. 11, 2010 2:25 PM EDT

Can someone please explain the rules and/or cultural conventions governing stoppage time in soccer? Does the referee actually use a stopwatch to keep a cumulative total for each half? Or is it just a rough estimate based on how much non-playing time he thinks has elapsed? And why is stoppage time kept a secret in games with three referees? Can anyone point me to something that explains not the official rules, but the actual normal conventions for how this all works?