I went and played at the University of Waterloo the other day, and the players on the varsity team are give rackets with plastic on the frame. Its similar to having a racket cover on just the frame.
With the training racket, they are supposed to practice their swing and if they do it correctly it should make a sound. The sound is like a boom and a woosh combined. I've been playing badminton for about 3-4 years now and learned pronation, wrist snap and all of that... however i could barely make the sound.
Does anyone have/know of a video where they demonstrate it or could give me more insight and reasoning behind this type of training and how it's done