I can't remember what the advantage is over the ball, except that you can adjust the height and the firmness of the spring. It also feels less awkward and takes less room. your legs aren't forced away from you, you can perch them under the seat for better control. Oh, and the ball can't swivel side to side the same way so you can do more dynamic postures to work out your core.
You need a firmer core I would think, to support your spine and abs under the strain of contractions.
I'm thinking there has to be an angle or pose to keep the spine aligned - or a set of muscles that needs reinforcement.
... Pain itself causes tension. Tension that can be nullified mentally. I think it's entirely possible that you at least partially counteracted the problems that pain caused last time - you handled the situation with discipline and fearlessness. It takes mental discipline to get into a proper mental state to not fight pain that can't be helped.
You have to overcome more than I did in labor. I avoided pain in my 4th labor because all was aligned properly and I entered a birthing trance where I could coach myself to release and relax myself into the peaks of each contraction with water helping to further dull any aches - my spine certainly wasn't getting pulled out of alignment. If it had been I would have needed a deeper trance or very well built up numbing suggestions like the kind needed for deep surgery to get total pain elimination. But even without total pain elimination, just good relaxation will take you farther and make you birth faster.
The good thing is that you are experienced. You will go farther because you will remember what soothed, what didn't and won't waste time trying what doesn't work. Plus you will birth faster regardless. Your body is better at it the second time around.
"I dont have back pain now, so I wont know if any of this works until labor..."
I just wanted to go over that point one more time to see if there's anything we missed.
Are you practicing making numbed out spots (on your arm, for example) with your mind and testing that spot? If you are, keep reinforcing that lesson. Make the anesthesia more powerful, deeper and move it around your body. Come up with personal trigger words for implanting numbness into your spine and abdomen. Maybe try using a plain alcohol rub, pretend that it's got a powerful anesthetic in it and have it applied via cotton ball to your skin as a numbing agent. Then test it with a pinprick. Compare it to an area that is not numb. Feeling pain is not failure, by the way, it just is feedback to show you if you need to work on your technique, because you will get it right as you think on it and continue your research. I believe that it can work for you or anybody else who is receptive to the idea.
Take advantage of every episode of pain or itching you experience. You also have the excuse to go lie down and rest when in pain anyway even if it's just a finger that's burned because you need to keep your stress hormones under control, so just go lie down and meditate if you can. Or do a waking suggestion you built up during hypnosis. You don't have to be lying down in a trance to trigger old suggestions to work. Whatever seems right to you in the moment.
The point is, you can know and you need to know beforehand. By testing with other types of pain. You don't need doubt if you want a powerful effect. And be good to yourself no matter what result you get. You deserve to be happy with your efforts whatever your results are, you are doing the best you can do!