The organ, up until about the time of Bach, had no ability to get louder and softer gradually, as a singer might while singing a phrase. When instrumentalists play music of that era and before they can be heard mentioning terraced dynamics, which is playing at one level and then another louder or softer.
For more about this in instrumental terms, look up Mannheim Steamroller - the technique, not the group.
But in England they began putting pipes in a box and adding wooden shutters to permit the organ to get gradually louder and softer. That's why the term "Swell" is used to explain the growing sound as the organ swells into the room.
If you can see pipes, they always play at one set volume, but pipes hidden in a Swell chamber can play at varying volumes, depending on how far the moveable expression pedal on the organ is pushed in.
The top keyboard often had pipes enclosed in a swell chamber, hence the name of the keyboard.
Many organs with one expression pedal like the Roland and Rodgers organs we are discussing, offer the choice of the organ being fully expressive (both manuals get louder and softer at the same time) or the Swell beign expressive and the Great being at full volume all the time.
But this does not mean that there are only two ways to play the organ with this control.
Often people feel that this means that the Swell can have an expressive solo voice playing, rising about the Great accompaniment stops. But there is another way to use this effect.
Set a full registration on both keyboards, and couple them together, but do not press open the swell expression pedal.
Play a hymn and on the last line open the swell pedal, giving a fuller, richer sound to end the hymn. It's a great effect.
Also register a 4' Fugara flute (VP4) on the Great with tremulant and the Swell Voix Celeste coupled to the great....the great may be used for a soft solo against the accompaniment on the Swell, or both hands may be played on the Great with the rise and fall of the sound of the Swell using the expression pedal creating a lovely effect.