No player shall leave the court during a match without the umpire’s permission, except
during the intervals (...)
However, court is understood as meaning the immediate playing area (usually reaching a couple of meters around the actual boundary lines, but not the players' boxes). No umpire will fault a player moving behind the back line to jump into the shuttle or rolling out sideways during a dive.
Even for more serious infractions (like going to the coaches or players' boxes in between rallies), it's in the discretion of the umpire whether and how to sanction the player:
Originally Posted by RTTO, §22.214.171.124 Players leaving the court
Any offending side may have to be reminded that leaving the court needs the umpire's permission (...)
If necessary, Law 16.7 (= yellow / red card) shall be applied. ...
(emphasis and comment mine). Note that exchanging the racket is a special case; RTTO §126.96.36.199 goes on as follows:
Originally Posted by RTTO, §188.8.131.52 continued
... However, change of a racket at courtside during a rally is permitted.
So you can't take a bathroom break, go to your doctor, or give an interview during or in between rallies (apart from the intervals of course) while your doubles partner plays on. As long as you are playing Badminton, you are fine.
Yes, can we clarify this? In tennis, using an angled shot, the opponent can be pushed so much out of the court side-wise, that he will attempt a straight return that travels outside of the net-posts. So in this case, the ball does not go over the net. In rare instances a point can be won using this "dream-shot".
I'm not sure if this scenario can happen in badminton because why would you return a shuttle that will land so obviously OUT, but if you wanted to, are you allowed to hit it outside of the net-posts but under net-height back in court? My guess is NO.