That's a small preview from Steve's DVD, however, here's a visual picture of what's happening in the cyclist's distal end of the upper limb (wow, now I recall everything I learnt in my biomechanics class). From everyday riding experience, I can say that putting too much pressure on the hands (upper body weight) is going to lead to tightness in the wrist, pinching in the nerves of the hands, and discomfort/pain in the shoulder and upper back. Another reason to check your bike position. This winter, I'm going to take my own sweet time to perfect my position.
Picture courtesy of Hughston Health Alert.