This article is from the Piano General Topics FAQ, by Isako Hoshino firstname.lastname@example.org with numerous contributions by others.
Player pianos use suction, not pressure, to work. As the
pedals are operated, air is pulled from the pump and the
entire stack is placed under a slight vacuum. This vacuum
operates a motor that turns the rolls in the spool box. The
piano roll has holes cut in them that when they pass over the
tracker bar, the tracker bar's holes are uncovered. A valve
is operated when the holes are uncovered that applies vacuum
to the striking pneumatic, which plays the note on the piano.