Set the bar across your upper back like a squat, feet about hip-width apart, knees with a slight, fixed bend that doesn't change through the movement. Push your hips straight back while your torso tips forward, keeping your spine long rather than rounding, until you feel real tension building in your hamstrings. Stop once your torso is close to parallel with the floor, or wherever your hamstrings run out of range without your back rounding. Drive your hips forward to stand back up, squeezing your glutes at the top rather than yanking your torso up with your lower back.