When I checked the mobility of the product the skirt fell off my waist and it seems the ball part didn't fit and came off, so not great.
For now, I moved the skirt down, fixed it, exposed the joint part and set it as a first aid measure. If you don't move it into a big "ku" shape, there's no problem. Normally, the lower back is firmly connected so that it does not come off easily, but there is also a possibility that the reception of the ball part is poorly shaped, individual differences? It would be nice if that were the case...