I may be wrong (I normally am), but I recall seeing a post or something on the internet. Its was to do with the height of the spine, and were the ball is struck, to optimise the response of a shot it should be in the middle and struck at the highest point of the spine. This was to do with the vibration from where the ball strikes the bat which goes to the back of the bat (spine) and then back to the face.
Without finding the thread or site I have no prof and I am by no means a scientist. I don't think it is the density of the bat but the volume of wood from the face to the spine.
You can then go into pressing - but I do not know much about that.