To push her ass against the upright she has to push against something else. Her feet are directly under her so she can push against the nails to push her body up. Since her feet are nailed to the wood, she can also push somewhat against the upright here.
To use her upper body to push her ass against the upright she still has to push against something else. In the pic, the weight of her upper body is somewhat transferred to her stretched out arms which then pull on her nailed wrists, and down to her feet. She's not sitting on the sedile so all hanging weight is distributed between the nails through her wrists and feet. Her arms are behind her. How can she push with them? She can only pull up. Unless her ass somehow suctions itself to the wood behind her, I don't think she'll get any support there.
This position seems to me very temporary. I see it more an instance of her pushing up with her legs against her nailed feet to relieve the strain in her arms and the pain from the sedile/cornu in her groin. When her legs lose their strength she'll drop back down to sit on the sedile and hang more from her arms.
If her body drops down in front of the sedile (it'll poke in her lower back) then she's either hanging solely from her arms or balancing between hanging from her arms and pushing up with her feet,
Any position she's in only trades off one unendurable pain for another. There's is never any relief overall.
That's it, the diabolical reality of the cross with nails. Fixed at wrists and feet, the weight is on the nailed wrists/arms/shoulders, and the nailed feet/legs. To favor one is to punish the other, there is no other choice. We often forget how important the position of the feet is. Too high and they are almost useless in taking weight or providing thrust to raise the body. Too low and again there is not enough flex to allow the dance. The only option, the only relief, is some form of misericord or cornu, that is, a seat or an impaling anchor. The victim takes the cornu into their body to provide relief against the nails, against the relentless horror of gravity on her limbs.
This illustrates a combined seat and cornu, I hope she found it comfortable