Along with many other things in this time restricted enviroment of ours, the reason for doing the long brief well before flying the exercise has been lost. On the principle that a student only retains about 30% of a briefing (at best), the idea was that he went away, re read the brief or the prepared exercise notes and got more of the plot fixed in his mind. The short pre flight brief was a just a check on his retention, not new knowledge, and then detailing how the exercise was to flown under that days conditions.