Human bodies have the same principles: feet to hips (Root Section), waist to chest (Middle Section), and shoulder to hands (Upper Section). Each of these main sections has three sub sections.
 

Upper Section:

Shoulder (Root)
Elbows (Middle)
Hands (Upper)

Middle Section:

Chest (Upper)
Heart (Middle)
Dantian <low abdomen> (Root)

Root Section:

Hips (Root)
Knees (Middle)
Feet（Upper）

The knack of the three sections: the Uppers move, then the Middles follow, and then the Roots chase after. For example , the movement of punch, the fist is the upper of the upper section , after you lift your fist , the elbow (Middle ) follows and then the shoulder (Root) chases after . Only in this way, you can explosive power smoothly. When you progress forward, life the feet, then the knee follows, and then the hip chases after. Thus the center of gravity of the body is moved forward, the ten toes can grasp on the earth stablely like Mount Tai.