What equine chiropractic is
Care directed at the horse's spinal column and pelvis, on the same premise as the human work: the connections between the body and the nervous system run through the spine, and the spine's mechanics matter to how the animal moves.
In its modern form, animal chiropractic is a young field — most of its development has happened within the last century. Practitioners who work on animals need specific training in animal anatomy and biomechanics, because the differences from human structure are not small.
What owners usually notice first
A change in the gait. Resistance to a lead. Difficulty with a movement that used to be easy. Soreness through the back. A performance horse that has quietly stopped performing.
Conditions with a mechanical or neurologic origin are the ones animal chiropractic addresses — chronic back and neck soreness, restricted movement, musculoskeletal pain that resists an easy explanation.
The performance horse
Horses used for racing, dressage, or pleasure riding are athletes, and they take the same repetitive load that human athletes do. Regular assessment of range of motion is preventative work: it's aimed at catching a restriction before it becomes a compensation and the compensation becomes an injury.
As with dogs: this is not veterinary medicine, and it does not replace a vet. Where surgery, medication, or a diagnosis is what the animal needs, that is a veterinarian's job.