How do swell boxes let organists make sounds louder or softer?

In a pipe organ, a swell box is an enclosure used to regulate the volume of sound produced by a specific division of the instrument. The box contains a set of pipes and features one side constructed with hinged, movable shutters that function similarly to Venetian blinds.123

These shutters are connected to a swell pedal located at the organ console. By operating this pedal, the organist can open or close the shutters, which allows them to make the sound from the enclosed pipes progressively louder or softer. This mechanism provides organists with a method for controlling volume beyond simply changing stops or switching between manuals.34