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.