Posture is about 10-15% of the reason for effectiveness. Best to have about 7-8 degrees spine tilt at time of release....this helps with throwing an upward trajectory (lowers the release point).
The action of the wrist/fingers is about 85-90% of the reason for effectiveness. ....let the posture and wrist/fingers do the work.
Rick, does Sarah intentionally try to spin the ball backwards with her thumb and fingers? It doesn't look like the "turn the knob" action typically described.