when i watch Cai Yun or LYD play at the net (when their respective partners are about to attack at the rear), they always place the racket quite up high. But strangely, the Indonesians don't do that. I don't quite get it. How high should I place the racket when I am at the net when my partner is about to attack at the rear?
not as low as your ankle/knee. Not above your head - you might block your partners shot! If you put it at net height I don't think you will find too many will argue against that!
Be careful! When your opponent is about to attack from the rearcourt, a little bit higher than the net may be ok, but if the attack is average, or you are in other situations at the net, having your racket higher than the tape can be detrimental. I am imagining a scenario where your partner plays a drop shot and your opponent comes forward to the net to play a net shot. If the net shot is reasonable, you risk hitting the shuttle into the tape. If your racket starts at just below the level of the tape, you racket can rise up and into the shot, with less fear of hitting downwards unintentionally. I am not saying the advice by coach LJB is wrong - it is a good starting point. However, I am issuing caution. There are times when having the racket head above the tape makes a shot more difficult, if the opponents shot is of a high quality. Good luck!
High enough to be able to kill a loose return or to be able to intercept, but low enough not to obstruct your partner's shot.