I usually refer people to this version:
Barbell Straight-back Stiff-leg Deadlift
The key is a straight back. The knee can bend slightly.
Here if Fred Hatfield's description on how to do it:
"
STIFF LEGGED DEADLIFTS
A lot of bodybuilders use stiff legged deadlifts to exercise their lower back. Because your lower back is more efficiently and effectively developed with back extensions, there is no need to do any other exercise for your lower back, and ESPECIALLY not stiff legged deadlifts!
Stiff legged deadlifts are particularly effective for developing your hamstrings (the back of your upper legs).
The traditional way of performing this exercise is to lower the weighted bar all the way down to your bootstraps while standing on a platform or bench with stiff legs (or knees slightly bent). In this way, it's believed, you'll get maximum effect on your hams. This may be true to a degree, but you're also going to unnecessarily expose your lumbar spine to injury. Those intervertebral discs down there come loose all too easily!
I believe that I've developed a better way. With barbell in hand, poke both your butt and belly outward. In this position, you look kinda like one of the "Keystone Cops" you see in the 1920s movies. This variation of stiff legged deadlifts has thus become known as "Keystone Deadlifts."
This seemingly strange position will prestretch your hamstrings because of the forward tilt of your pelvis the position entails. Then, while maintaining this position, slowly lower the barbell to around your knees, keeping the bar close to your legs during the descent and ascent.
You must NOT go more than an inch or two below your knees. By the time you reach your (slightly unlocked) knees, your hip joints have fully flexed, and further lowering of the bar is accomplished ONLY through hyperflexion of your spine -- a NO-NO!
You will feel a decided "burn" in your hams and glutes when keystones are done correctly. You should feel virtually no discomfort or stress in your lower back. If you do, experiment with the movement until you feel no discomfort at all.
The nice thing about doing stiff legged deadlifts this way is that you can use a far heavier weight, thereby getting better adaptive stress applied to the target muscles. All without any low back trauma at all!
One more important caution: NEVER do this exercise explosively! You'll risk pulling a hamstring or blowing out a lumbar disc. Bodybuilders are well-advised to steer clear of heavy deadlifting movements, as they are potentially dangerous to the lower spine."