I don't want to get in a habit of copying/pasting Dave Tate all the time, but hey, "if it ain't broke don't fix it." So here it is:
"Sumo Style Deadlift: Use a moderate stance and a close hand grip. To start the lift, you will rock into the bar, and the hips come up fast toward the bar. This requires a strong back because the legs lock out long before the bar is completely locked. The most common style is with the feet very wide, out to the plates. The lifter should not lower the hips any more than necessary. The back must be arched to the extreme. Most important is to push your feet out to the sides, not down. Why? By pushing down with a sumo or wide stance, your knees will come together, which is the most common mistake in the sumo. By pushing the knees out forcefully, the hips will come toward the bar fast making for a favorable leverage, placing most of the work on the hips, legs, and glutes. TIPS: Donít stay down too long. It will destroy the stretch reflex."