This short video show the ICR flow for a casting style that some see as a lower hand dominate style. That conjectures the bottom hand is pulling back on the butt of the rod while the upper hand is functioning as a "fulcrum" to support the rotation axes of the rod.
The ICR flow in this video shows that is not what is actually happening.
You can see the rotation at the start of the cast is dominated by the left shoulder rotation. During the power application phase the ICR moves to a point between the two hands. That would indicate the rotation near RSP1 is coming from a force couple produced by both the upper and the lower hands.