in a mechanical stop action, each stop control operates a valve for a whole rank of pipes. when the organist selects a stop, the valve allows wind to reach the selected rank. this control was at first a draw stop knob, which the organist selects by pulling (or drawing) toward himself/herself. this is the origin of the idiom "to pull out all the stops". more modern stop selectors, utilized in electric actions, are tilting tablets or rocker tabs.