Here's an example of Ken Griffy ....
Keep watching those two frames being toggled back and forth in the middle of the swing.
That's the main engine. That's the "hands being taken off the shelf with the main engine" (i.e., we "walk-away-from-the-hands", "we move forward and place the hands on the shelf" ... and then we use the 'main engine' to "take the hands off the shelf"). It's a combination of torso 'rotation' and 'lateral side bend' (and yes ... also lumbar lordosis, or arching of the lower back).
I believe that you do arch the lower back as well as tilt or lateral bend sideways. I believe that is the movements that take place. You need both.