but wasn't very sure if spinal rotation had any purpose other than orientation
It (I consider this trunk rotation) helps define the overall size of the backswing/loop. If you want to add a ballistic element to this, there's two ways to do that.
1) You can visualize a vertical stripe on the ball, and that your racquet face is making contact with that stripe.
2) You can imagine your torso twisting around your waist.
In both cases, what you want to look for is your loop to be "higher." Regardless of the size of the loop, the contact point should be about the same, though.