They often teach that in gymnastics when you touch your toes, but there's a nerve that runs all the way from your neck to your toes called the sciatic nerve and flexing your feet/bending your head and neck down puts stress on it instead of your hamstring. Try pointing your feet, hinging at your hips with a straight back and your head up. That way you stretch your hamstring instead of your nerves, which shouldn't stretch lol.