When swinging a hammer you need the wrist, elbow and shoulder to accomplish it with power.
This is a similar statement:
Kenpo students are usually aware of coordination of upper and lower body, but Tai Chi and stillness requires distinguishing between substantial and insubstantial; that is the amount of weight on each foot, when one leg is full and the other empty, or partially so, and which corresponding hand movement is substantial or insubstantial; that is, the amount of extension or force of each hand, arm, shoulder or back movement. With this one must be aware of where movement emanates. For instance, most Kenpo students begin the punch with the fist moving forward. But the actual movement must first come from the elbow; yet the elbow cannot move without the shoulder behind it; and, this is where most Kenpo students get their speed.
Your best moves come naturally, why you need to train in others...