Almost everyone knows what a bellows is: an accordion-like contraption made with two boards hinged on one side and covered with cloth. When it is closed, air blows out a pipe and when you open it air is sucked in. This is the principle the foot pumps work on. When you foot-pump, each pedal is attached to a large bellows. Operating a pedal, forces a bellows to open and it sucks air out of a reservoir. If you attach a tube to the reservoir, opening the end allows air to be sucked in. The keys and the motor are activated by small bellows called pneumatics. These are normally open. If suction is applied to them they snap shut. These pneumatics cause nearly all the mechanical action that takes place. Each key is played when a pneumatic snaps shut. The roll motor has pneumatics equipped with valves that open one after another making the motor shaft turn.

Piano keys are teeter-totters. Striking a key makes the far end rise causing a complicated mechanism to flip the felt hammer against the strings. The pneumatic stack that has the individual pneumatics is in place below the far end of the keys. When a pneumatic snaps shut an attached push rod rises striking the bottom of the key. The perforated roll passes over a brass tracker bar which has a row of holes, each corresponding to a key. Each hole has a tube that goes to its valve and pneumatic. When the moving paper uncovers a hole air is sucked into the tube that may be thought of as a "trigger" tube because it opens the valve to its pneumatic.

When the piano starts and the roll paper covers all of the holes, suction from the reservoir goes to every valve in the pneumatic stack and through the valves and trigger tubes to every hole in the tracker bar, and the valves stay closed to their pneumatics. When the moving paper roll opens a hole on the tracker bar, air is sucked into the "trigger tube" and it’s valve opens to the pneumatic which snaps shut to play the note.