There are three power sources in the baseball swing. With a player who has a correct, 1,2,3,4 kinematic sequence where the hips initiate rotation followed by the torso, lead arm, and lastly the hands, there will be three places where separation can take place.
The first place of separation occurs between the pelvis and torso and is commonly known as hip-shoulder separation. Once the heel strikes the ground and the rotational component commences and the pelvis rotates away from the torso, there is a stretch created through the oblique abdominals that leads to elastic energy. Most power hitters get to around 45 degrees of separation between the pelvis and torso.
Hip Shoulder Separation
The second power source comes from generating torso-lead arm separation. This is created by retracting the shoulder blade towards the spine during the negative move holding the shoulder blade in place during the positive move. This sets the table for the torso to fire to catch up to the pelvis while the hands stay back due to the retracted shoulder blade which creates separation between the torso and lead arm. Often, flexible players with excellent torso-lead arm separation look like they are arm barring, but this ability to get the bat back is a positive characteristic.
Torso-Lead Arm Separation
The third power source comes from the separation between the hands and bat. This happens by hinging the wrists in radial deviation. The barrel of the bat remains behind the hands until right before contact when the wrists snap delivering the barrel to the ball in a motion similar to hammering a nail.