I have pointed out many times, for a batter to generate his maximum bat speed, his swing mechanics must accelerate the bat-head around the entire swing plane. This means the batter must first accelerate the bat-head rearward (or back toward the catcher) before he starts to accelerate the bat toward the ball.
Below is a clip showing the pre-launch mechanics of Barry Bonds. Note that Barry starts his pre-launch movements with his bat out in front of his head. From that point, for the next few frames, the bat-head is being accelerated in an arc back toward the catcher.
Bonds PLT Mechanics
There are many coaches who still teach their batters "not to release the bat-head too soon," keep it back until the hands extend.- However, most coaches that have studied the swings of the best hitters and understand rotational transfer principles, know that to attain maximum bat speed, the bat-head must accelerate around the swing plane right from initiation - this means first back toward the catcher.
Note: After much study and discussion, most coaches who have posted to this Site now agree with the above principle. However, that was certainly not the case when I first presented this finding from my study of the baseball swing on the Site.
Although we agree the bat-head is accelerated rearward, there is disagreement in defining the mechanics great hitters apply to generate that acceleration. I have long maintained that the batter induces the rearward acceleration by shoulder adduction and forearm pulling the top-hand back around a slower moving (or more stationary) bottom-hand (THT).
There are coaches, such as WT in his post below, who believe there is no "pulling back" of the top-hand. They believe it is strictly the rotation of the shoulders and the lowering of the back-elbow to the slot that accelerates the bat-head rearward. I fully agree that these are important factors of the mechanic. However, I would like to make a few points for your consideration.
If the bat-head is accelerating rearward around a slower moving bottom-hand, then the top-hand must also be arcing back around the bottom-hand. How much bat-head acceleration occurs as the shoulders rotate and the elbow lowers depends on the direction of force the top-hand applies to the handle of the bat. Since the bat-head and top-hand are rotating rearward, maximum acceleration is generated when the pulling back and rotation of the forearm causes the top-hand to apply force in the same direction the bat-head is accelerating - rearward at that point.
Great hitters generate great bat speed because they are pulling back on the handle with the "fingers" of the top-hand as the forearm rotates and lowers during initiation. Average hitters also lower their elbow to the slot. However, they produce only average bat speed because their top-hand is pushing with the "palm" on the handle instead of pulling back and rotating their wrist. Applying a force with the palm means the force is directed forward which drives the knob forward, instead of accelerating the bat-head rearward around the swing plane. This result is much less bat-head acceleration and the batter quickly falls behind the powercurve.
It is the natural tendency of hitters (including most pro hitters) to only think of the foreward acceleration of the bat-head. Therefore, it is more natural for them to drive the bat-head forward during initaition by pushing forward with the palm of the top-hand. Since the back-shoulder and hands (as a unit) are starting to be rotated forward, it feels unnatural for the top-hand to be pulling rearward. Only the best hitters accelerate the bat-head around the entire swing plane by first pulling back and rotating the forearm as the elbow lowers.
Coaches, we can keep it simple and just teach the student what is more natural "rotate the hips - slot the elbow - take the hands to the ball - palmup/palmdown." That is easier for the coach and less confusing to the player than teaching the mechanics of the great hitters. -- As for me, I find the results is well worth the extra effort.