What NextLevel said. But there is, of course more. Efficiency.
Usual when a lower level player goes across their body and the elbow goes up, there is a curved plane that the bat face goes through. And that curved shape is inefficient. The blade face should stay in one plane.
I am not sure the words give the right image. If you have your arm straight and you bend your elbow to 90-degrees, the hand will describe half a circle. THAT IS GOOD. If you do that so the hand is always in the same plane THAT IS GOOD.
Say you had your hand on a table and you did that so that your and was gliding on the surface of the table as the elbow went from straight to bent: that would be good. Now say instead of a regular table the table was a drafting board and the face of the drafting board was at a 45-degree angle and instead of your hand, it was the FH rubber of your racket and it glided over the surface of the drafting board never moving closer or further away from the surface. That would be a good stroke actually. If you had a little pen on the tip of he blade, it would draw a quarter of a circle. But the bat face would be moving in the same plane across the surface of that table at a 45-degree angle to the ground.
Whereas, when your elbow rises and your racket ends up lower than your elbow, the racket has done something much less efficient from a movement standpoint and so there is A LOT of wasted effort in they stroke that ends across the body and lower than the head.
The angle of the plane for each stroke should be adjusted based on the spin on the ball. But the blade face should stay as much as possible in one plane. One plane is much more efficient and much more how the biomechanics of the skeleton would want you to create the stroke.
The curved plane of a stroke where the elbow lifts and the hand curves down, where the player is trying to arc around the ball (from under, up the back of the ball, over the top, down the front of the ball) is a harder movement on your forearm and shoulder, there is no way you could achieve the same bat speed, and it takes much more effort and you get much less from it and it would be a much less accurate shot.
Okay, there is much more than this too. But, now I have to stop.
LOL.
BTW: great idea for a thread by NextLevel. Thanks.
Sent from the Oracle of Delphi by the Pythia