The hamstring is comprised of three distinct muscles the semimembranosus, semitendinosus and biceps femoris. The semimembranosus is on the medial side of the leg and the biceps femoris on the lateral or toward the outside relative to the semitendinosus. When you are training these muscles in a prone position it is valuable to know what occurs physiologically.
Lying on your stomach in a neutral position and internally rotating your foot during a prone leg curl increases the activity of the medial hamstring close to 10%. By the same token externally rotating your foot during the exercise can reduce the muscular activity in the same muscle about half the above value - 5-6%.
Good information to know and understand when recovering from a knee injury, hamstring injury or when Getting Strong.