The Automatic Pedal piston, or Bass Coupler button, as it's known, is a very useful and handy coupler for the pianist who's been asked to move to the organ and doesn't have command of, or maybe just doesn't want to use, the pedals just yet.
Abbreviated "A.P." (photo), this device couples all the Pedal division stops to the main (Great) manual and makes them playable there on the lowest key among those depressed, while at the same time silencing the pedal keys.
When this coupler is in operation with all manuals coupled to the Great, it makes it possible for every stop in the instrument to "talk" through this one keyboard without using the feet; being reversible, the second push of this control turns it off.
Typically this coupler is only operable on the bottom 32 notes of the main manual; the Pedal stops would not sound above middle g1 of the Great manual or on any Great keys other than the lowest one held down.
By doubling the bass note this way and bringing the Pedal stops into play on the Great manual, this coupler gives a spread sound, color, and sonority that can be very helpful to the pianist working with the organ for the first time performing hymns or other organ music for 2 hands only [See menu bar, Free Stuff].