Forward folds with the feet roughly sitting bone-distance apart will stretch the central portion of the hamstrings. These include Paschimottanasana (Seated Forward Bend), Uttanasana (Standing Forward Bend), and Halasana (Plow Pose).
Taking the legs wide will bring the stretch into the inner edges of the hamstrings. Along the way, the adductors (inner thigh muscles) will get involved, too. That’s fine, but see if you can feel the difference between the two groups. Poses that stretch inner hamstrings include Upavista Konasana (Wide-Angle Seated Forward Bend) and Prasarita Padottanasana (Wide-Legged Standing Forward Bend).
You can stretch the outer hamstrings by taking your legs closer to the midline or turning your toes in in standing forward folds. You can also feel them release in Parsvottanasana (Intense Side Stretch) and Parivrtta Trikonasana (Revolved Triangle Pose). If you have a very tight iliotibial (IT) band, you may feel sensation there, too.
Using a strap, as shown in the video, will help you find and release each of the areas in the hamstrings group. Lie on your back with the strap around the ball of your left foot, right leg bent at the knee or straight along the floor. As you stretch your left foot up toward the ceiling, you’ll feel the central hamstrings stretch. Use the strap to pull the foot in until you feel a pleasant intensity. After 10 breaths or so, move your left foot over to the right to find the outer hamstrings stretch, holding for another 10 breaths. Finish by moving your left foot slightly out to the left for 10 breaths for in an inner hamstrings stretch. Stop with the left foot hovering over the left edge of the mat so this doesn’t become primarily a stretch for the adductors deep in the inner thigh. If you need to deepen the pose to find the stretch, move your left foot up and into the space over your left shoulder.