I'll take some of the blame here.  Twenty-one years ago, on p. 67 of my
book "Player Piano Servicing and Rebuilding," I said:

  "The pneumatic should not be hinge-bound due to excess glue
  or incorrect hinging.  In fact, a pneumatic which is covered
  correctly with cotton cloth should be flexible enough to fall
  open and shut under its own weight."

Several people have called my attention to this statement over the
years and asked if it is really true.  With regular pneumatic cloth,
it is not, unless the pneumatics are unusually large.  The cloth and
hinge should provide only minimal resistance to closing, but they do
add resistance that prevents pneumatics from falling shut on their own.

I apologize for the concern that my statement has caused.

Art Reblitz