I am the OP and here is my reasoning and observations
If you hit anywhere, including the center, on the strings while swinging low to high, you will get topspin.
This of course assumes the strings are vertical or closed [upper edge ahead of lower edge]
But if you are also rotating your forearm CW [supinating] and thus the face moves
from closed to less closed to square/vertical through the ball,
for the purpose of getting more spin with the gear effect, it will tend to fly on you.
However, you can counter the CW rotation with a
CCW torque created by hitting below the center.
The supination CW rotation of your arm/wrist exerts a Force
on the ball and the ball exerts a reaction Force on
the strings which yields a CCW torque about the long axis
from the butt to the tip of the frame.
This is due to Newton's 3rd Law ...... Action => Reaction
This balancing of the CW and CCW torques/rotations leads to more stability and consistency.
And the supination provides more topspin by the gear effect than simply swinging low to high.
It does feel funny at first and you get less power, But the benefits are worth it.
If you minimize or eliminate supination thru the ball, like hitting flat,
hitting below center is not necessary.
Those who think this is all BS should look at Bautista-Agut hit a FH
And he does it on some BHs too.
He does not do it on his return of serve; he hits that one flat.
Slow it down to 1/4 th speed with settings in the lower right.
You will be able to see it much more clearly.
Be patient and look for at least 4 - 5 minutes.