Next time someone calls you for an illegal serve, or for any other rule infringement - ask 'em to say what's wrong. If they can't tell you, how can they say it's wrong. If they won't tell you, what you did must be OK and they're just being an a$$. BTW I always carry a copy of the rules in my badminton kit bag (Still haven't agreed with some of my fellow club members about the 'double hit rule' tho.' )
Briefly, the head of the racquet has to be below your wrist and technically below your bottom rib. Obviously it's a little difficult to catch some of these. Also, you cannot stand on the lines of the court when you make the server. And you also need to have both feet planted on the ground. Those are some of the main things to look out for illegal serves. Use them to your advantage to catch others as well.
Below the wrist simply prevents u from hitting an overhead style shot. Bottom rib pointfore.ca? one of my coaches told me the contact point between the racquet and the bird has to be below the bellybutton. Lol nebody kno wut the technical definition of waist is?
different sports and disciplines may have other standard for "waist". but for badminton, the standard (technical or not) for "waist" in badminton is the lowest part of the ribcage.