In the three pictures the hip is always in a different position. In picture one, the hip is tilted forward. This creates a hollow back. You can reinforce this by also pressing through the upper back and moving the head backwards.
The second picture shows a neutral hip position.
In the third picture, the back becomes a hump. The abdominal muscles pull inward.
Alternating the positions is like a warm-up. You develop body awareness. When you are on all fours, it is easy to move the hips. It becomes more difficult when sitting or when you want to move into the splits from a standing position.
The fact that you have to go into a hollow back to do the side split is an important insight. Without this hip movement, the legs cannot be stretched straight to the side.
As is often the case, it feels like the body is deep in the position. But when you look at picture one, reality quickly catches up with you. There's still something there.
Baddha padmasana, April 2023
Baddha padmasana is a pose of the final sequence in Ashtanga Yoga. Here is practiced in a sitting position to make a hollow back. In addition, the shoulder blades are pulled together, the chest opens.
This week I will focus more on the side split. It is much more difficult than the forward split.