Sumo Deadlift

Trains the Hamstrings.

How to do it

Set the bar over the middle of your feet and take a wide stance, feet well outside shoulder-width with your toes turned out.
Push your hips back and down and reach for the bar with your arms inside your knees, hands about shoulder-width apart.
Your shins should be close to vertical and your hips lower than in a conventional deadlift, with your chest up and your back flat.
Push your knees out in the same direction as your toes — this opens the hips and lets you get down to the bar.
Take a breath, brace, and take the slack out of the bar before you pull.
Push the floor apart with your feet and stand up, keeping the bar against your legs the whole way.
Finish standing tall with the hips forward, then lower by pushing the hips back and bending the knees once the bar passes them.

Letting the knees drift inward at the start. Pushing them out toward the toes is what makes the wide stance work; knees collapsing in is the position failing before the bar moves.
Setting the hips too low, as if it were a squat. A sumo deadlift is still a hinge — the hips are lower than in a conventional pull but not as low as a squat, and starting too low means the hips rise first anyway.
Letting the bar drift away from the shins. Same rule as any deadlift: every inch forward increases the leverage the weight has against the lower back.
Choosing between sumo and conventional as though one is correct. They load the same muscles in different proportions — sumo asks more of the adductors and quadriceps, conventional more of the erectors and hamstrings. Hip structure decides which feels better.