According to ASMI:
The greater the maximum external rotation in pitching, the greater distance there is to accelerate the hand forward and consequently the greater the ball velocity.
Max ER is not something that is gained by stretching, unless you're coming back from an injury, or have a deficit of some kind that a Dr. or PT has assessed and is treating you for. MER is a dynamic measurement that is the result of a combination of the rotational velocity of the upper trunk, the amount of rotation at the shoulder joint, the amount of movement of the shoulder blade, and the amount of extension in the upper spine (arching the back).
We've seen too many people out there cranking back on a pitcher's arm, trying to improve his MER, when all they're actually doing is stretching out the joint capsule and creating an unstable shoulder joint. If there isn't a health issue, then you improve ER by improving the efficiency of the pitching mechanics, beginning with the hip/upper trunk separation.