I've been asked by my friends, who are less fortunate than I, who ride straight going stock to explain what I mean by "Setting a gait". To explain what it means to set a gait, first we must discuss the Naturally Gaited Horse.
A "Naturally Gaited Horse" is a breed of horse that has been specifically bred for a smooth easy gait. These include your Tennessee Walking Horse, or the Paso Fino, or your Missouri Fox Trotter. These animals have been bred for many years to produce these easy gaits. However, Some horses are naturally more talented than others. While they are all of them able to do their natural gaits, it requires a bit more from some to learn to do it whilst carrying a person on their back.
This leads us to the setting of a gait. Plainly put, to "set" a gait means to have them hold the desired gait indefinitely. So, when we are setting a gait, we are encouraging the equine to not only hit the desired gait, but to hold it until we ask him to change. This requires just as much from the rider as it does the equine. Your horse must be taught to understand when he is in the "right" gait. Whether it be a fox trot, a running walk, or a rack, you must be able to communicate with him to let him know he's in the right gear. This also requires that the rider know the difference between gaits. It does him no good if you don't understand gait. So, first learn your gaits, then you can teach them to your horse. Happy trails!