The Coupling is actually the placement of the lumbo sacral (LS) joint and its configuration. To begin with you need to read and understand this paper by Dr. Deb Bennett:
This explains the ring of muscles and how they work to help a horse better carry himself. If you can strengthen the ring of muscles, any horse can improve their way of going and carriage, even if they do not have great conformation, are built down hill or have a weak LS area.
Next I will give examples (in photos) of various LS issues