There are two big reasons to use sumo deadlifts: To build a powerful posterior chain and to lift as much as humanly possible. Training for the former means focusing on glutes and hamstrings, along with upper and lower back muscles. A good hinge pattern hits all of these.
But let's address the second reason. For high-level powerlifting competitors (those trying to lift as much as humanly possible), it's fine to use any technique within the rules that'll allow you to lift the most. If this means a squatty sumo, so be it. Adjustments made for a competition squat, bench, and deadlift are to maximize leverages to get the highest one-rep max total possible.
But those adjustments are often different from proper training technique, and they're even counterproductive to maximizing the training effect for strength and hypertrophy. They often create injury risk, accepted by competitors trying to win. Think exorcist-level bench arch or round-back deadlifts.
For those of us sumo deadlifting for the other reason, let's dispel the two biggest myths and fix the flaws in this lift to make it a safer training tool that'll get us better results.
A lot of lifters think their feet need to be spread as wide as the plates will allow, but that's a good way to drop a plate on your toes. Really, your feet just need to be outside of your arms and hands in order to be a sumo. Take a look.
Granted, a wider stance feels great for some and has leverage advantages which shorten the distance from ground to lockout. But some people's hips just aren't mobile enough to get into a wider position. Wider isn't necessarily better.
If you get your legs just outside your arms, this is called the "hybrid sumo" or "semi-sumo" and it's a perfect option to gain the benefits of deadlifting.
Some clients I've worked with can't get into a conventional stance (arms outside of legs) without rounding their lower back. This is caused by a limited hip flexion range in a narrow stance, which makes the semi-sumo a better solution.
A semi-sumo stance can also prevent knee valgus or inward collapse. This is another common problem that arises when placing your feet too wide. In this case, your knees don't remain stacked above your feet because your hips can't externally rotate wide enough. Or it's possible you aren't focusing on creating torque with the hips to pull the knees into the wider position. This may lead to knee pain and injury over time if not addressed.
Deadlifts are a hip hinge pattern. There will be some people who can squat their sumos and lift more weight while experiencing zero pain. For the rest of us, let's make a simple adjustment and get a better effect.
When lifters set up with low hips that jump up before the weight leaves the ground, this usually indicates a loss of back and core tightness. Your hips and hamstrings are seeking the tension you failed to create during the setup.
There's no problem with beginning in a squat position then pulling the hips high and getting wedged in before lifting, but issues arise when the hips jump in an uncontrolled manner as the lift is initiated.
So, when setting up, firmly grasp the bar and pull it into your shins. Pull the shoulder blades together and create a flat neutral spine. Then elevate your hips until you feel coiled tension in your hamstrings. When you feel this tension, your hips are like a loaded spring ready to snap forward and generate more power. In a recent seminar, Tony Gentilcore described this process as creating a "lifter's wedge."
Your sumo deadlift shouldn't look like a squat. Your hips shouldn't drop to the level of your knees. Your feet don't need to be as wide as possible, and your lower back and knees shouldn't hurt as a result.
Now that we've got those essentials out of the way, let's tack on a few other notes:
Pull the slack out of the bar.
You can do so by pulling on it without enough power to lift it off the ground. This should create tension through your body and a stable spine. Then initiate a smooth but powerful push with your hips forward toward the bar. Lock out by squeezing your glutes under your ribcage and avoid hyperextending your lower back.
Control the negative (lowering) portion.
Just remember that heavy weight will come down quickly. Some lifters can touch-and-go smoothly between reps with tight form. Others need to reset every rep to maintain good form. Avoid bouncing the weight between reps. Use the style to best maintain excellent form, maximize training effect, and walk away healthy.
Avoid rounding at the back.
Some lifters argue that a rounded back is safe and makes them stronger. They're usually referring to some roundness of the thoracic (mid-upper) spine, done by skilled and experienced powerlifters to lift more in training and competition.
A practiced core brace with a rounded thoracic spine and flat lumbar is often a good risk versus reward tradeoff for such experienced lifters, but there should be no need for anyone else to set up with a rounded upper back. And I'd avoid ever lifting with a round lumbar spine outside of high level competition for rare max attempts.