Instructions
Set up at a squat rack or where you can attach a powerband from a height. The thicker the band, the more assistance you will have. Position your ankles under a fixed padded support (and in this case kneel on a bench. You can also perform this exercise from the floor).
Place the powerband around your upper torso and place your hands up in line with your chest with your palms facing outward. Lower your body with hips straight, controlling the descent with your hamstrings as low as possible. Only when you can not control it any longer, touch down onto the bench/floor, with your hands on either side of your body. Once your hands make contact with the bench/floor, control the remaining descent with the assistance of your arms. Bring yourself back up by pushing off with your hands, just enough to assist your hamstrings to raise your body to the upright kneeling position. Repeat.
Tips
For a more advanced exercise you can perform this without the resistance band.