This has become a classic hamstring and lower back movement over the years for one reason, it works!
This has been a staple in powerlifting, bodybuilding and sport training programs for years and will continue to be. The stress (when done right) that can place on your hamstrings is by far the greatest you will ever feel.
To begin stand up with a barbell and lock your lower back in. By this I mean push you hips back and pull your chest up. You should feel all your body weight on your heels before you begin to lower. As you lower the weight begin by pushing you hips back until you feel your hamstrings stretching. If you feel all the stress in your lower back you have lost your arch and are not pushing back but falling forward. Keep the weight back on your heels and return by flexing and driving your hips forward.