In the American-made players the keys fall as the whippen is lifted.
This holds true as well for new players installed in the USA-made
pianos.

There is a piece of wood that runs the length of the keys going left
to right just past the front rail pins that is hinged to swing up under
the keys and stop their movement when desired.  Some old players have
this bar connected to the lever that opens the bottom door to the
pumping bellows so it comes active when the door is opened.  It's very
easy to remove.

Don Teach - Shreveport Music Co.
Shreveport, Louisiana