Swing Up or Swing Down? 

Creating proper swing plane is not as simple as "swing up" or "swing down". Hitters are being bombarded with information about how great hitters have success. There are countless stories of Major League hitters transforming their swings and careers by "getting the ball in the air". The information is good. We know all great hitters have a positive attack angle (the vertical direction of the barrel at contact). We know that hitting the ball with a launch angle of 10-30° produces more hits and specifically more extra base hits. How hitters interpret this information is not always ideal.
At the point of contact, just after the barrel starts to work up towards the baseball, the bat is ballistic. The hands are no longer communicating force to the bat. We see that when great hitters swing, the majority of the force that gets applied to the handle of the bat is happening when the bat is working down and behind the baseball.
If we dump the back shoulder and the middle of our bodies, our swing happens "all up" which is just as bad as swinging "all down". Our time on plane is just as short and our margin for error is gone. So is our ability to lengthen our swing out towards the field and adjust when we are off time.
Create positive attack angles, create positive launch angles but do it the right way. Swing down to swing up. Allow your posture and hip hinge to create a natural arc that will get the barrel working down and behind the baseball then turn upwards around the hands.