Hinge, reach, and hold. A deep standing hamstring stretch.
Square your hips toward the floor instead of opening them to chase height.
Folding over your standing leg while the other leg rises behind you puts the standing-leg hamstrings into one of the deepest stretches you can load with just bodyweight, since your whole torso hangs as the counterweight. Keeping the hips square is what keeps the stretch honest, the moment they open sideways the hamstring tension drops. It earns its place in cool-downs and mobility sessions, especially for lifters whose hinge depth is capped by tight hamstrings.
Mostly the hamstrings of the leg you are standing on, plus the whole posterior chain of that side. The lifted leg is just leverage, the standing leg is where you should feel it.
No, height of the back leg is the least important part. Keep both hips square to the floor and the standing knee straight, and take whatever lift the hamstring allows. Height comes as flexibility does.
After, or on rest days. It is a deep static hold, and long deep stretches right before heavy lifting can dull output. Save it for when the work is done.
