I know, I know, there are lots of other threads on improving smashing technique, but I have a specific question that isn't mentioned in those. I was watching a video of Tony Gunawan smashing from the end of the court looking directly down the line and I noticed how much he twists his back in his wind-up. It looks like his arm is about to twist right off his shoulder. His elbow actually breaks the 'plane' of his shot, i.e. it's more than 180 degrees back from the direction the smash is going to be hit. I've sinced noticed how much all the top players draw back before a big smash and it all seems to involve a lot of tension in the waist and shoulders. My question is this: what's the best way to stretch your back safely and to practice this way of smashing without injuring yourself? Is there a good drill or something to improve smashing this way?