Did anyone notice the shift in the stack fairly early on? The entire stack rocked by about a foot or two, accompanied by a loud metallic boom. It's apparently something to do with locking the stack in place, but that must be pretty disconcerting to watch happen in person. I'm a little surprised that such an energetic motion is acceptable. Rocking the entire stack must involve a lot of energy. That can't be good for any of the hardware involved.