With the modern swing players are taught to get the bat and hands flat early to get on plane and have a chance for a good slight uppercut which you want.
However this can lead to laying the bat back too early, flattening the hands too early and a long and slow swing as well as still failing to get the slight upswing.
Here you can see the correct hand path with votto:
I tracked the rear elbow, front elbow and hands. You can see the front elbow goes forward 2-3 inches and then goes up. How much depends on the pitch height but it will go up almost always. The hands go down (and slightly forward) to around the rear nipple to mid trunk depending on pitch height and then go levelt o slightly up, like a flat lying „J“.
What you want to do now is not supinating the hand early and laying back the back. Instead keep the bat up and the palm facing the pitcher until the elbow starts up.
I created this sequence. I like allowing the wrist to supinate to help turning the barrel but not before the front elbow starts to work up and the hands and rear elbow down. Basically want that supination to happen where the turn oft he „J“ turns into the straight (only straight from side view of course). That way energy is stored and it really snaps flat instead of laying back.
This creates a pivot point around the hands that is going down and then turning up. If you lay the bat back early the laying back overlays with the down arc oft he hands which means you don’t get a good path. But if you delay the down arc oft he bat until the hand path flattens out which is caused by the fron elbow working up you get a tighter and quicker swing arc and actually a better upward path.
Now here is a drill: lead arm swing off a tee: To promote connection I put a towel between front elbow and chest. Make sure it is up against the chest like when doing that shoulder muscle stretch and not down in the arm pit so the elbow can be up. The bat should point up and the arm is flexed, then you load by slightly protracting the front scap and allow a few degrees of lead elbow extension.The towel should fall out after contact.
What you do now is putting the top hand against the barrel and force keep the barrel up until the front elbow works up (in real swing the top hand staying pronated for a moment hast hat role). When the elbow goes up, the bats wants to go down, but resist for a few inches and force it to point up. When the elbow has worked up like 2 inches you release the barrel and allow the barrel to snap downward to get on plane. Finish high and forward with direction.
First have the Tee slightly behind the back hip. This sounds strange in these days, but stay with me for a moment: You want to hit the ball with the barrel arcing downward slightly or at the low point down to level. Later then you put the tee to the front knee to foot and like magic it goes up. Now you have a tight and quick snappy swing that still gets on plane early.