Based on Extremis from when i took him apart, the bar that connects the lower and upper part of has four detents in it. You can feel it click the one time but it doesn’t click after that and he just springs back. I’m not super familiar with the inner workings on bodies with reverse abcrunch and torso rocker so it was new to me, but with Extremis Iron Man the bar slides into a soft rubber piece that also houses the torso rocker joint. Getting the heat/hot water to that part and rocking the joint forward seems to let that soft rubber piece form better to the bar somehow. I’m not 100% why it works but it works.