I keep studying about the pelvis and the hip joint. I like to introduce my version of its explanation from mainly kinesiological point of view.
The pelvis angle affects the head/neck relationship. This means that we won’t accomplish the best practice of the primary control if we leave the disadvantageous pelvis angle. One reason is that the spinal column will not be aligned adequately if the angle of the pelvis is disadvantageous. Since the sacroiliac joint is not moved much normally, we can consider the pelvis as the bottom of the spinal column. The rising angle of the bottom of the spinal column will definitely affect the alignment of the whole spine above (with the limitation of the moment of force that our body can afford). The cervical spine is the part of the spinal column, and the angle of the cervical spine will be affected by the pelvis angle. The muscular condition in the neck will then be altered and usually end up with more tension.
Similarly, the muscular condition around the hip joint usually synchronizes with the muscular condition around the neck. If the muscular contraction degree around the hip joint is excess, the person will often tight his/her neck muscles excessively. This synchronization comes from our way of supporting the body. What we do when I say supporting the body is that we are supporting the torso against the legs and the head against the torso. So, if there is a person who tries to support his/her body excessively, this person will give excess muscular contraction both in the neck for supporting the head firmly and in the hip joint for supporting the torso firmly at the same time.
To the contrary, the practice of the primary control (i.e., the head forward and up, the back widen and lengthen, inhibiting the excessive neck tension) may not affect the condition of the pelvis and the hip joint and may leave their disadvantageous condition. This happens more in the AT beginners. So, I fix the condition of the pelvis and hip joint as well as the head/neck relationship for sure I get the ideal condition of the whole body.
Now, if this is true, then we had better know what is the ideal condition of the pelvis. We had better know what the adequate direction for the pelvis control is, just like the direction for the primary control. I have an idea for the direction of the pelvis, which is: Position the buttock backward until getting a sense of receiving a little weight at the front side of the thighs, and be the state such that the buttock is ready to move backward (for not holding the hip joint too firmly). I have described this in a past post.