Rubbing noise from engine backside

Do not rotate the engine backwards or you will ruin the starter sprag clutch and it will be a costly repair.

Never turned the prop backwards for more than a couple of centimeters. But it doesn’t very natural.

What you are hearing is the brushes in the starter motor turning backwards against the commutator.

As you already know, it does Not have any immediate damaging effect.
You are probably accelerating the wear on the brushes but not much more.
It is not a recommended thing to do, so, Knock It Off !!!  (Stop Daarmee !!!)?

That’s the sound of money flowing out for replacing the sprag clutch.

This is not a rhetorical question. BUT...

How is the Starter driving the Crankshaft Forward through the Sprag Clutch mechanically/physically different in any way from having the Crankshaft drive the Starter in Reverse through the Sprag Clutch?

Once the Clutch is locked up under Torque, does it matter if the torque is coming from the inner ring or the outer ring?
The starter applies many times more force to the clutch than reverse hand propping does.

When the engine experiences a Kickback and turns the Sprag Clutch in reverse;
The problem is Not that the Clutch is turning in Reverse.
The problem is that it reversed directions in 3 milliseconds and the Starter is still trying to drive it forward.
It is then under the extreme forces of being between the Proverbial Rock and the Hard Place.

I am not asking if hand turning the prop in reverse might be detrimental to the Starter or anything else.
but, how Specifically, might it be detrimental to the Sprag Clutch?