Not sure where you heard this from AT ALL. Studies found that it SHORTENS the external rotators (despite the player feeling stretching - this is caused my increased elbow extension) increases, the torques on the elbow skyrocket, AND as distance increases ball speed DROPS.
"It has often been hypothesized that long-distance throwing is beneficial to the throwing athlete for increasing flexibility, ball speed, arm strength, and endurance. A previous comparison of adult pitchers with high versus low ball velocity demonstrated 3 kinematic differences. Namely, pitchers with high ball velocity had greater maximum shoulder external rotation, forward trunk tilt at the time of ball release, and lead knee extension velocity. For longer throws, the current study found greater maximum shoulder external rotation but less forward trunk tilt. Furthermore, the current study found no differences in ball velocity for various throw distances, and approximately 10° of knee extension from foot contact to ball release for all throws. Thus the current study did not find greater similarity between particular distances of throws and the pitching mechanics of pitchers with high ball velocity.
In another study investigating changes within individual pitchers, several characteristics correlated with greater ball velocity. Kinetic values near the time of ball release (elbow flexion torque, shoulder proximal force, and elbow proximal force) increased with pitch velocity. For pitches with higher ball velocity, at the time of ball release, pitchers displayed decreased shoulder horizontal adduction, decreased shoulder abduction, and increased forward trunk tilt. The current study showed no differences among throw types in ball velocity, kinetics at the time of ball release, shoulder horizontal adduction, or abduction. Forward trunk tilt decreased with throwing distance. Thus, the current study did not indicate that particular throwing distances were superior in training to increase ball velocity.
The current study did find greater range of motion (maximum shoulder external rotation), speed (angular velocities of the pelvis, upper trunk, and elbow), and arm torque (elbow varus and shoulder internal rotation) in long-toss, which indicates that these throws may be beneficial in training. However, long-distance throws also produced changes in throwing mechanics at foot contact (up hill trunk tilt and foot position) and at ball release (forward trunk tilt and front knee flexion). Furthermore, maximum-distance throws produced the greatest elbow and shoulder torques, without any change in ball velocity, making these the least efficient throws, as they produced the most torque for comparable ball velocity. The benefits or detriments of long-toss cannot truly be determined without prospective studies comparing performance and safety between groups trained with and without long-toss."
The actual writing of explaining this was written by Brent Porciau -- not me. (Don't want to be a plagiarist)