These two exercises, stiff legged deads and Romanian deads, I think have sort of been wrapped around each other in myth and shadows
Everybody seems to have a different take.
Not to mention that some people teach the conventional deadlift based on it's
difference from these other two. So you get this kind of thing like "conv. deads are more like a squat than the stiff or Romanians".
I actually do think that the stiff legged deadlfit has basically had elements of the Romanian deadlift molded into it in "recent" years and so that has really created some confusion. Ian King, oh he of the spinal flexion addiction, says it was the influence of later Europeans teaching a flat back and the popularity of the Romanian deadlift that did this. I think he is probably right on that. But I think, of course, that he is WRONG, that the Europeans were wrong about flat back (naturally arched) being more healthy and superior. Of course they were right. But that is a different story.
The stiff legged deadlfit actually has the bar traveling out from the body and there is no 'butt back" in it. Originally it WAS done with spinal flexion. I think most people doing stiff legged these days are doing a sort of morphed version of a Romanian and the original stiff legged. So it people seek to explain it by the degree of knee flexion allowed in the two movements.
The original Romanian deadlift was basically a deadlift done from the hang position with the knees and hips always in slight flexion (you never lock out) and lowered to just under the knees are a little lower depending on flexibiltiy.
If you know much about deadlfiting it will look like the lowering portion of a good deadlfit and then you will also see that no knee flexion is required of most (provided they aren't very inflexible) to get to this position. You just keep the back set, flex the hips by moving the butt back, and lower the bar down to just below the knees are a little lower. Only if you sought to lower the bar more than this would you need more knee flexion. And if you artificially sought to produce more knee flexion, which I have seen, then you are doing a very unnatural thing.
Lots of people pretty much explain a stiff legged deadlift as the same thing as that. It's not. The bar does not hug the body and it probalby was most likely done originally with completely straight legs and rounded back. If only slightly rounded back. You are basically bending over with the bar and picking it back up in the same way.
There are some die-hards who still do that. And many others who call what they are doing stiff legged deads but are doing a sort of version of a Romanian. The semi stiff leggeds ARE done to place more STRESS on the lumbar erectors. Whether rounded or not, moving the bar away from the shins is going to do this. While Romanians will train the heck out of lumbar endurance (and overall back endurance) while tageting the glutes and hamstrings greatly.