I guess a benefit of spending a long time at an element (in this case arch body rocks) is that you get a lot of time to perfect the iM.
Concerning the iM (Standing Trunk Circle): as you rotate around your hips, your spine should more or less be straight, correct? I've had an issue where I notice that I'm twisting more in the thoractic spine, not from the hips. To correct that I focus on just rotating from the hips while keeping a straight spine. To unlock that rotation (from the hips only) I've noticed that the hips do not stay still, but actually counter-rotate in the direction of the spine (not a huge amount, but there is movment there). In other words as I swing around from the back to the left, my hips will move to the right. So it's not about your hips staying fixed and just the upper bopdy rotating, but rather the upper body rotating one way, and the hips another (again not to the same degree that the upper body moves). It seems to be what is happening in the video.
So... am I doing this correctly?