He writes on a white board on the table, and what he has written gets reflected off the mirror over his head onto the mirror on the wall, the effect being just like a blackboard.
如此一來，他在桌上白板上書寫的字，經由頭頂上的鏡子反射到牆上的鏡子裡，就如同在黑板上書寫一般。