Stiff legs, deep stretch, all hamstrings.
Keep your back flat and the dumbbells close, rounding the spine to chase range is how you tweak it.
Keeping your knees nearly straight turns the hinge into a long lever that puts the hamstrings under a deep loaded stretch at the bottom of every rep, which is the position they respond to most. The glutes finish the job, driving your hips forward to stand you back up. Dumbbells make it easy to keep the load tracing your legs and to control the lowering phase, so it is a natural hamstring builder for home and gym alike.
An RDL keeps a soft knee bend and usually stops around mid shin, the stiff legged version keeps the knees straighter and chases a deeper hamstring stretch. Same movement family, this one just prioritises range.
Until the hamstring stretch is intense but your back is still flat, and not a centimetre further. That depth is different for everyone and it grows as your flexibility does.
Usually the dumbbells have drifted forward away from your legs, or your spine is rounding at the bottom to chase depth. Shorten the range, drag the bells down your thighs, and the hamstrings will take the work back.
