In reply to Roger Waring, I cannot give a definitive answer because
I have never seen a Kastner action but I can suggest that what he finds
may be correct.

In my Strauch action in an Ajello 88 note player piano there are two
separate interconnected suction reservoirs.  The left reservoir has a
light spring and the right reservoir has a much stronger spring.  The
theory is that this gives a more progressive effect than a single
reservoir of equal combined volume.  The Kastner arrangement sounds
similar.  An additional feature on the Strauch is that there is a
rocking lever acting between the two reservoirs.  The closing of one
will add to the spring opening force of the other.  This is alleged
to aid sudden fortissimos by a sharp increase in pressure by the foot.
I am not convinced!

Happy pedalling,

Nicholas Simons, England