Every bit of it is ALL in the upper back and core bracing. I have focused harder on my training and actually using my core and learning to brace properly harder this passed 4 months than I have focused on even trying to get stronger. To be stable you have to brace perfectly you can't just flex the core you have to learn to try and rip your belt apart. What I mean about that is that you always hear about pushing into your belt a cue that worked for me is I am literally trying to inhale as much air as I can into my stomach and then I literally try to break my belt lever. I want to literally do everything I can to physically try and break the belt by pushing out and into from every single angle possible. Take a similar approach with the upper back. You will see in the video even with 225 I look down and bring my chest down as it helps me feel my stomach and really push out as hard as physically possible then you will see me pushy head back into the bar then, I literally in every single sense of the world squeeze the bar with everything I have and then try to physically bend the bar across my back trying tk ake the ends touch skde posckets if I had them. This locksy lats in very hard and pulls my scapula down and slightly back putting me into a very strong and stable position. Then from here I push my hips though on the unrack. If when you unrack your Shoulder Griddle Rises up at all then you were near tight enough before tracking in the upper back. The unrack should be a complete hip and lower body movement.
As for staying upright I had to move my stance out in order to this and slow my decent down when the weight gets stupid heavy. Once again I'd rather sacrifice speed down than give up even a half inch of proper bracing and upper back position.