It originated during Run Amok recording sessions due to the natural tendency of the band and the producer to both get stuck talking about and quoting The Mighty Boosh for hours on end while they were supposed to be recording.
'The Boosh Rule' initially was a rule that was set to prevent this - no talking about The Mighty Boosh until you were finished tracking for the day.
In the time that has passed it has since spread and come to refer to any procrastination or lack of attention to the immediate task.
Meeting participant 2: Oh hey did you hear that there's a new program guide service starting up?
Meeting participant 1: Cool, no I didn't - when does it go online?
Meeting participant 2: It should be starting soon, pretty cheap I hear.
Meeting participant 1: Neat, forward me the URL.
Meeting participant 3: Hey! You two! The Boosh Rule.