Utthita parsvottanasana

To move into utthita parsvottanasana with that blocked SI joint is painful. That is it's painful when I move into this pose from standing position. That's the classic movement. One steps the feet apart from each other, then one turns to the right sight. In the meanwhile the hands are in prayer position behind the back. Then one lowers the upper body to reach with the chin the shin. During this movement the lower back has to carry the weight of the upper body. It's not only this, What I read it's much more than the body weight, it's four times more due to the leverage effect.
This is also why it's recommended to lift things very close to the body. This prevents the back from damage in the long run.

My version these days: I bend forward, I reach my feet with my hands. I can support this movement while putting my hands on my legs. This takes away weight from the back. Then I step back. The smaller the step the more difficult becomes this asana.
In classes I often heard that one shall move the weight to the back leg. With my hands on the floor I can support this shift. When I feel comfortable, I move my arms behind my back. See picture three. A warning: the shoulders are usually a weak part of the body. Take care not to injure the shoulders when using them to leverage this asana.
Yet from there it's so much easier to take the hands behind the back in prayer position.

All forward bending asana improve when the abdomen are engaged (bandhas).

I want to relax in a pose. It must feel good. Not at the beginning, but after some time. If this is not possible, I go to the position, that feels good.

On chronic pain: I was very astonished when I read that one speaks of chronic pain if someone has pain for only a few years.Then the brain shall sent messages to the nerves that sustains the feeling of pain even though the cause for this pain doesn't exist anymore.
The recommendation is to ignore the pain. It's recommended not to talk about it, not to think about it, not to read any literature about it. One shall even not do any exercises to relief the pain. Forget about it, is the advice.
This is what I tried yesterday. Taking picture distracted me from the pain, too.
I got that point.
Nevertheless I think in my case, the cause, the blocked SI joint is still there. I can locate it. I got to know the movements when it hurts. However. I'm looking forward to the rolfing sessions next week. I must believe in this treatment!