[Kevin Sevcik]

I think a lot of their stack stability comes from that claw. If you look close in slow-mo (cause they're moving too fast otherwise) you can see a shiny bit below the red claw at the back. When they've lifted 2 totes, the claw briefly releases and drops to the top of the totes, then closes again with that shiny bit at/below the top edge of the top tote. It's important enough that they actually pause all that insanity until the claw re-engages. So that claw isn't just an RC grabber, it's also for capturing the top tote. Holding that tote with a little downward pressure would definitely make a much more stable stack.