Sam Byrd has taught me the idea with Sumo Pulling is to try and lock everything where it needs to be and then Squat the weight up. You want as little lumbar spine movement as possible. Get set up, chest high, press thought the heels and if all is perfect then you will have very little lower back movement. However it is very hard to master and a lot people me, you, and Sam him self all have the same problem when the weight is a true max effort. That's why I said look at some one like Dan Green who is damn near perfect.