The rotation should be 100% natural. It should be the outcome of all the energy and force you generated before you released the ball.
If you are forcing the leg rotation, then you may not be generating alot of force toward home plate.
If your back toe is dragging and making somewhat of a straight line toward home before it lifts off the ground, then you are in good shape and generating energy force toward home. But your line is probably squiggly or even at a 45 degree angle. Long straight line toward home is the goal.
In summary, you should generate enough force that your are almost falling toward home plate. See Tim Lincecum for example.
See his toe drag and his leg rotation....it is 100% caused by the energy he created before that.