It doesn't happen too often, but it does happen once in a way, and I am almost always left asking, "Why?" The receiver requests (gestures to?) the server for the shuttle to be changed. Server agrees, requests (gestures to?) the umpire for the shuttle change. OR The server requests (gestures to?) the receiver for his acceptance for the shuttle to be changed. Receiver agrees, and server requests (gestures to?) the umpire for the shuttle change. So, both players are agreeable to changing the bird. Mostly, the umpire just gestures to the server to go ahead. But sometimes, the umpire asks the server to bring him the shuttlecock, then inspects, and refuses permission for the shuttlecock to be changed. When this happens, I have almost always heard the commentators wonder why the umpire did not give permission. Perhaps there is an explanation. Can anyone help?