During tournament-level badminton play I don't have a problem with odd serves, but at the high school varsity level a different kind of backhand serve has surfaced. Players will basically perform a quick pull back followed by an immediate flick for a rapid serve. I've never seen this type of serve before in competitive play before, so I checked up on the BWF regulations, but it simply says that the motion must be continuously forward (nothing about the back-forward double motion)
The reference to "continuously forward" motion indicates that the server is not allowed to take multiple "fake" swings before hitting the shuttle. Doing so might unbalance their opponent and give an unfair advantage to the server, making a game quite one-sided.
So once the racket has been drawn back, it can only be swung forwards once, as it hits the shuttle.
What is left to preference is how fast they swing back and forwards, which can catch the opponent out if they're not used to it. If you carefully observe someone with an unusual, but legal, serve a few times, you should be able to anticipate what kind of serve they are doing and not be caught out.
Laws of Badminton, pp. 5-6
The movement of the server’s racket shall continue forwards from the start of the service (Law 9.2) until the service is delivered (Law 9.3).
This rule, together with rule 9.1.1, ensures that the service action must be one continuous movement with no double-action feints. To be precise, you are allowed to serve with either of these actions:
- One backswing immediately followed by one forwards swing
- One forwards swing on its own (no backswing)
Facing a Rushed Service
The server shall not serve before the receiver is ready. However, the receiver shall be considered to have been ready if a return of the service is attempted.
When you're facing someone trying to rush their service, it's best to simply not move. If the serve lands in, then you simply ask for the point to be replayed because you weren't ready. If the serve lands out, then you take the point!
Rushing the service is a cheap tactic, and can be easily countered if you stay calm and don't move until you are ready.