The term "stops", as used here refers to air valve controls used on pipe organs to allow or interupt the flow of air from the organ's bellows or blower motor into the various wind chests for the particular ranks of pipes it controls. In other words, each rank of pipes in a pipe organ is designed to produce sounds of a specific tonal color. A "stop" contol for each rank is built in to the organ console to allow the organist to select which rank or ranks of pipes are activated as he or she plays. Generally, these are in the form of "draw knobs" with an engraved heads providing the names of each stop. The stop knob is drawn or pulled out to allow air to flow into the windchest channel under the rank of pipes it controls. Then as keys are pressed, smaller valves under each pipe controlled by those pressed keys allow the air to actually reach the toe of the pipe causing it to sound. A stop knob is pushed in to shut off air flow to each rank of pipes. Obviously, the more stop knobs you pull out, the louder and more powerful the organ's sound becomes,

as you play. The phrase "pulling out all the stops" is a way, based on the pipe organ and the organists, of saying: "Give all you've got!". Or, "Turn it up to 11!" if you prefer.