Constant-tension hamstring stretch from the cable.
Keep your back flat and the bar close. Lead with the hips, not the lower back.
Running the RDL off a low cable changes the resistance profile, the stack keeps pulling as you hinge, so your hamstrings stay loaded deep into the stretch and your glutes still meet real tension as you drive your hips through at the top, where a barbell version eases off. The lighter absolute loads also make it a friendlier hinge for higher-rep work. It earns a place for constant-tension hamstring volume when your spine has already done its heavy lifting for the week.
Barbell for heavy strength work, cable for constant-tension volume and a hinge you can rep out without loading your spine as hard. They're teammates, not rivals, many lifters run the barbell early in the week and the cable version later.
Until your hamstrings pull tight with a flat back, usually somewhere below the knees. The stretch is the target, going deeper by rounding your lower back just moves the work somewhere it doesn't belong.
Set a soft bend at the start and keep it fixed. If your knees keep bending as you lower, you're drifting into a squat and your hamstrings lose the stretch that makes the exercise work.
