Near-straight legs put all the load on your hamstrings.
Keep the bar close to your legs and let your hips travel back, not down.
Keeping the legs nearly straight removes the quads from the pull, so the hamstrings have to handle the descent under a deep loaded stretch while the glutes drive the hips forward to finish each rep. The bar tracking close down your legs keeps the load where it belongs, in the hips rather than the lower back. It is one of the best hamstring builders with a barbell, sitting alongside the conventional deadlift as the stretch-focused sibling.
They are close cousins. Both hinge with minimal knee bend, but the stiff-legged version typically keeps the knees straighter and chases a deeper stretch, often from a greater range. If you feel a strong hamstring stretch with a flat back, you are doing the important part right.
Not necessarily. Your range ends where your hamstrings run out of stretch with a flat back, and for most people that is somewhere below the knee, not the floor. Going lower by rounding your spine gains nothing.
Usually the bar has drifted away from your legs or your spine is rounding at the bottom. Drag the bar down your thighs, push your hips back rather than down, and shorten the range until your back stays flat.
