A coupler is a linkage between two Banks, possibly with a change of octave as well.
Adding a coupler
To add a coupler, first tap the spanner icon to start Configuration mode, then the plus icon at the bottom, after the last coupler.
This will then prompt you set up the coupler, which has the properties listed below.
When the new coupler is added to the screen, it will appear last in the list. Drag and drop the couplers to reorder them.
Changing a coupler
To edit a coupler, first tap the spanner icon to start Configuration mode, then the pencil icon next to the name of the coupler.
Input and coupled banks
E.g. to couple the Great to the Swell, select the Great as the Input Bank, and the Swell as the Coupled Bank.
Just in case of confusion, note this actually appears as Swell to Great back on the main screen. This is the convention for labelling of couplers on organs that dates back centuries.
Also, the input and coupled backs can be the same bank - to add a transposition...
Transposition
Adding a transposition will let you create a superoctave, or a suboctave coupler.
If the input and coupled banks are the same, this will simply cause all stops to play their notes twice, in both octaves.
Midi channels
As mentioned on the stops page, each active stop is allocated a Midi channel, and the Midi specification has a 'baked in' limit of 16 channels. Unless you have multiple output devices, the typical maximum number of stops you can pull out is actually 15, because channel 10 is reserved for percussion.
However, using couplers is quite 'economical' with the 16 channel limit in Midi. If the Great is already playing, and you connect the Pedals to it, any notes played on the Pedals use the same channels that were already allocated to the Great.
Similarly with a superoctave, or suboctave coupler, no new channels are needed to play the extra notes. Both of these are great techniques for building up the organ sound.