Theory from an amateur
Watching some pros and some people on YouTube. I have this theory that turning your shoulders as fast as you can is a big key element.
Let's pretend we do not have arms, and we are just trying to turn our shoulders as fast as we can.
At address: push that left shoulder down as hard as you can while pulling that right shoulder back and up towards the sky as you can.
At the top:
Now reverse this and push* that right shoulder down and thru the impact zone and pull that left shoulder up and back towards the sky .
Without even engaging the arms we can get our shoulders to impressive speeds.
One of the keys here is that the pushing and pulling are independent of each other. You must use your muscles to focus on one (pulling) and other muscles to focus on the other (pushing).
Now engage the arms and figure out where they go in your swing.
Musings from an amateur... lol
Is this a sound excercise?