This is a killer hamstring movement that my good friend Todd Brock used to live by when trying to build his sumo deadlift. By taking the stance out you change the stress on the hamstrings making it more accommodating to a sumo deadlift and wide stance squat.
To perform this one get into a wide stance and sumo deadlift the barbell to the start position. To lower drive your hips back as hard as you can while keeping you back arched. Lower yourself with your knees slightly bent but all the stress still on the hamstrings. When you can lower any further (or hit the floor) return to the starting position by driving your knees out and chest up.