I have to disagree with the above statement. "Internal Rotation" is the notion that the shoulder complex, upper arm, lower arm, hand, and wrist wants to internally rotate to the center of the body (i.e. the position that the body would be in if you are standing upright facing forward and your arms are hanging relaxed to the side of your body naturally).
The whole idea that "Internal Rotation" only happens at or after the release of the ball is a fallacy. In fact the whole process of arm whip is using internal rotation as soon as the shoulder and arm reaches the 9 o'clock position (directly behind you to 2B), the hand will be "palm to the sky" and as the shoulder and arm continues forward they are continuously "internally rotating" into AND through release. Internal Rotation is what is generating arm whip and ultimately velocity to the pitch.
If Boardmember is around he can articulate the process better than I can.
This is all a good description, but there is one fallacy that needs to be noted about natural arm position. The natural hand position when facing forward, arms at the side, is palm forward, like in a biceps curl. The palm touching or slapping the thighs is a pronated position. This hand shake position is already pronated 90 degrees, with the radius moving over the top of the ulna. Soon after that 90 degrees of pronation, the elbow and shoulder have to compensate for continued IR.
This in no way involves our debate about 12:00, 09:00 palm to the wall or palm up issue. This is strictly about the bone orientation at or after release.