Xue Sheng
All weight is underside
Yu Bei Shi: Preparatory Posture
From Chen Xiao-Wang's Chen Style Taijiquan: Family Transmission. Translation by David Ho
The upper body should be upright, but the chest should not stick out.
The head should be straight, as if a light object has been placed on top of the head and you're trying to prevent it from falling off.
Lips lightly closed, teeth barely touching, chin slightly tucked in.
Relax the shoulders, but don't consciously try to sink them.
Chest muscles should be relaxed, the shape is neither concave nor convex.
Relax your waist and let it sink down naturally. This section of the spine curves naturally forward and downward, just relax and retain this shape, and the qi will sink down naturally.
Knee should be bent, kua (pelvic region) should be relaxed, straight, and centered.
The requirements listed above applies not just to this posture but also to every movement throughout the routine.
The above is from here