The foundation of Xingyiquan is it's stance keeping practice called San Ti Shi (also known as San Cai) , which means "Three Body Posture" or "Trinity Posture." It is the very core of training and develops many of the qualities essential to the development of martial ability.
The "three bodies" refers to the three phases all together, i.e. heaven, earth, and the human being. It corresponds to the head, hands, and feet in Xingyiquan. These phases are again divided into three sections.
Head - The position of the head is the key to the alignment of the whole body. When standing, the head is gently lifted upwards allowing the entire body to release tension and align itself properly with gravity. The chin is slightly tucked down and in while the head is pulled back and slightly up, as if hung on a meat hook. The Eyes are level, looking straight ahead and into the distance. Sometimes the eyes will be closed. The ears "listen" behind you and to the sounds of the body. The facial muscles remain relaxed; one should not wrinkle the forehead creating tension between the eyebrows. The tongue is curved upwards, touching the roof of the mouth and thus connecting the Ren and Du meridians, allowing the circuit to complete and the qi flow smoothly.
Body - The body should be centered and balanced. The shoulders drop and "get behind" the arms as the chest is relaxed and sunk slightly inwards. The shoulders should never lift upwards and should align with the hips. The buttocks are relaxed and have a sinking feeling. "Get into your legs" by pulling the tailbone slightly forward and under. This roots you better to the earth and straightens out the spine. The testicles should be lifted. As the body moves forward, the head and shoulders should reamain on the same horizontal plane.
Hands and Arms - The arms and hands are relaxed and held in gentle curves. They should never be fully extended. The fingers are separated and "shaped like hooks," allowing the qi to flow to the ends of the fingertips unimpeded. The hands are open and the palms deep. The elbows should feel heavy (with the mind) and remain dropped, protecting the ribs. "The hands never leave the heart, the elbows never leave the ribs." The index finger of both hands should be on the same vertical plane as the nose, or your centerline. The bottom hand should be at the navel or Dan Tian area.
Feet and Legs - The knees are slightly bent, never passing the vertical line which passes through the tips of the toes. Your weight should be in the back leg in a 70/30 distribution. This may vary a bit depending on the style. The feet grip the ground as if you were trying to pick up the ground with your toes. They should be visualized as twisting inwards and down like the powerful roots of a tree, gripping the ground - rooted, but ready to move without a thought.