What makes a serve legal in badminton?

• Server and receiver stand in diagonally opposite service courts, not touching the lines.
• Some part of both feet of the server and the receiver stays on the floor, still, from the start of the serve until it is delivered.
• The serve starts with the first forward movement of the racket head, and that movement must continue forward without stopping.
• The server releases the shuttle without adding spin, and the racket hits the base of the shuttle first.
• The whole shuttle is below 1.15 metres from the floor when hit.
• The shuttle's flight is upwards from the racket, over the net, to land in the receiver's service court if not returned.
• No undue delay once both players are ready. Pausing after the racket has been drawn back counts as a delay.
• The server must not swing and miss.
• The server cannot serve before the receiver is ready; a receiver who tries to return the serve counts as ready.

Without limits the serve would be an attacking shot hit downwards from close to the net. The height and upward-flight rules make the serve start the rally, not end it.