Internal Rotation (IR) is basically pitch release range of motion (ROM). The more degrees of IR, the more velocity producing potential. IR is elasticity in its highest pitching form, but is a subconscious movement of energy.
Meaning, energy loads and transfer rates create IR's range of motion, rate, and motion. A pitcher cannot consciously improve IR directly, as it is a byproduct of many things gone right or wrong.
SUPPORTING INTERNAL ROTATION OF SHOULDER
Since Internal Rotation is an explosive energy release, it puts strain upon the throwing arm joint axis points (shoulder and elbow). To ensure Internal Rotation arm injury risk remains low, support is required. Although pitchers can experience increased IR rates, if left unsupported - injury can occur.
Additionally, command suffers big time when Internal Rotation of arm is left unsupported because dynamic balance becomes misaligned - initiated PRR. This is seen predominantly in max effort pitchers and "run and gun" ballistic throws of weighted baseball throwing. It is important to train BOTH support and Internal Rotation of shoulder when trying to gain more velocity