I often used A4 paper rather than than a board to brief an exercise.
The student can just listen to what you have to say, all the little detail extras can be sketched out and the student can then take it with them, so that if they wish, can re-write it at a later date.
All this board briefing is a throw back, i assume, to a CFS classroom full of students.
Once rubbed of the board, that masterpiece is gone for ever!