Every posture is based on Tadasana, even Sirsasana. That means, to whatever degree possible, we are looking to be aligned as if we were in Tadasana even when we are upside down!
Here is a picture of me from the side in tadasana.
You can see a line running down from the center of my ears my ankles. Everything is in a straight line.
Your job is to keep that even when you raise the arms over the head. In yoga we want to learn to raise the arms without bending the trunk. The arms should move freely in the shoulder socket and the ribs, belly and pelvis should remain still.
This should be true even when you are upside down in Headstand.
Can you see how this student’s hips are tilted forward, the buttocks are backward and the lower ribs are flaring forward. There is too much bend in the lumbar spine. This is called “Dumping into the low back”. Interestingly, the student will often feel this in their neck as they try to compensate for the backward movement of the buttocks, not in their low back. (The site of the pain is not the cause of the pain.)
This may also be caused by a compensation in the torso because the arms are overhead. When the arms are raised overhead, if there is any restriction in the movement in the shoulder girdle, the lower rib cage will flare forward. This causes the buttocks to move backward. The body is compensating to find balance. You often can’t see this yourself.
Here’s a picture of a perfect Sirsasana. Can you see how the buttocks are moving into the body, not sagging backwards and the thighs are pressing back? These two actions create a lock: Mula Bandha, or root lock. These are opposing actions which create stability in poses, especially in balancing postures like Sirsasana.
What do you look like when you are in Headstand? Do you know?
There are things I don’t like about teaching online, but one of the things I do like is that I can take screenshots of students in poses and I can show them what I am talking about in terms of alignment. It is a very useful tool because often we think we are doing a pose in alignment when we are not.