|Bent knees break the stretch along the back of the legs and a cocked neck with rounded spine adds stress to the muscles, not stretch.|
Pulling on the heel buckles the knee and rotates the hip outward in its socket. This releases the tension needed down the back of the leg to gently stretch the ends of the muscles away from each other.
Lifting the hip cheats the stretch at the top of the hamstring by sending the leg up and forward. Leaning in with one rib also deprives the muscle of feeling and finding its length.
Bringing the leg to the upper body is far less effective than bringing the upper body to the leg. The back of the knee should be down and flat before reaching over to the side.
Pulling on the toes exerts a false flex in the ankle and prevents the body from learning how to move one end away from another. The same goes for the bend in the knee. He should use his muscles, not his hands to create the stretch.