Most people stretching their hamstrings are really stretching their sciatic nerve. The hamstring runs from the bottom of your butt to the back of the knee.
Rounding your back to stretch the hamstrings makes no sense. It does not improve hamstring flexibility but it does tension the sciatic nerve.
Single leg hamstring drops w/ ball.
For now, this one if the grand daddy of the hamstring drop family.
Set up with your feet under something that will not move. A dumbbell rack works great but not all are low enough to the ground.
I have used a standing hamstring curl machine to perform hamstring drops when the DB rack was not an option. See below.
Set up with your feet anchored under a dumbbell rack or other solid structure that will not move. Focus on bracing the abs, staying tall throughout the motion and not bending at the hips. Perform the hamstring drop with only one leg on the way down and both legs coming back up. Try to keep the emphasis on the hamstring, not the arms and shoulders. The upper body is definitely assisting the movement, but the hamstrings should be the focus. Enjoy.
This will punch your hamstrings right in the mouth. Set up with both knees bent, feet flat on the ground with a slider under one of your feet. Lift the foot that is not on the slider off the floor. Perform a single-leg bridge with the other leg. Once at the top of the motion, straighten the knee by sliding the foot away from the body. When the knee is extended, keep the hips on the floor, bend the knee and start again with the single-leg bridge.