When punching or throwing a strike in Muay Thai, I have heard that the punch should not be 'telegraphed'; meaning that the opponent should not be able to understand what you are going to do. When throwing an uppercut the fist must be brought downwards from the guard position to make a significant motion upwards that gains speed/momentum before impact. My question is how far downwards/backwards should it go? Down to the belt level? The height of the solar plexus? Or as far back as behind the hip? And does the chest move with the fist in the direction of the setup?
The fist should move as little as possible. The power of the uppercut comes from slightly dipping in the knees while turning the hip and then pushing from the hip. The elbow shouldn't move behind the body at all. The movement should look a little like the elbow is fixed at the hip and being pushed by the hip rotation/thrust. Only at the very end the arm moves forward by itself. This way you keep your guard up for most of the technique.