When executing a sumo deadlift most beginners will start the pull by not pushing the feet and knees out to the side, causing your hips to come up much too fast. But now there is a drill to prevent this. It is called the seated deadlift.
How do you execute a seated deadlift?
First, pick a chair or box to sit on about the height where your hips are when starting the bar off the floor. Next, while sitting on the box, place your feet in the same position as your normal start.
Then, push your feet forward so the shin is angled forward not vertical. Grab the bar with an arched back and pull into a sitting position. This will cause the bar to push your knees outward as the bar is pulled into the seated position.
Then, with the help of a partner, have them place a hand on your sacrum and one on your upper chest. Their job is to push the sacrum forward and the chest backward to a super-arched position.
Do two or three seated reps. Then on the last seated rep, pause for about two seconds, then stand up. This should make your hips go forward coming toward the bar as your upper body is in an arched position, which is correct.
Yury Belkin is the one to watch. I found this out while watching the great Russian deadlifters and how they start the weights off the ground. It looks perfect at first, but right before starting the bar off the floor, they bring their hips even closer to the bar.
This is how it should be done.
The seated deadlift will do the trick.