There are always a lot of mind games around the service procedure in badminton. Last week I played a guy who twitched his foot every time I was serving. I found this very distracting at the critical point of the stroke. Is that legal?
According to the Laws of Badminton, 9.1.3:
Some part of both feet of the server and the receiver must remain in contact with the surface of the court in a stationary position from the start of the service (Law 9.2) until the service is delivered (Law 9.3).
It's unclear what you mean about twitching his foot, but it sounds like it might be legal and might not be depending on the particular action of the foot.