When Spanish missionaries were making their way into California in the 18th century, the vaqueros (cowboys) came with them. The Spanish Empire granted vast tracts of land to loyal subjects, which were the basis for the Californio ranches and lifestyle common until the mid-19th century.
Ranches raised range-bred beef for Mexico and other markets. The cattle were half-wild and dangerous, requiring a fast, well-trained horse that could intimidate an individual cow, turn it back from the herd, separate it for branding and other handling, and do it all effortlessly.
Over time, the Californio vaquero developed a system of training the working cow horses that became famous for its elegance, precision, and difficulty of training the horse. The roots of these methods are in European dressage, a system to train horses for war. Adopted by the pre-Moors and Moors in Spain, and transferred to the Spanish conquistadors, the Californio methods created horses so sensitive to their riders’ signals they were known as “Hair-trigger” or “whisper” reined horses.
At the time, a finished reining horse (as it was called) required at least seven years to train: three to four years to train the basics in a bosal hackamore, then at least a year carrying both the bosal and the high-ported spade bit (named for the spade-shaped port which was from 1-3″ high) to help the horse learn how to carry the bit, then several years refining techniques in the spade until the horse was a “made” reining horse. The training could not be done by just any Californio, and reining horses were valuable because of the difficulty of training and scarcity.
A finished reining horse could be controlled and directed with minute movements of the fingers of the left hand, which hovered above the saddle horn. Because of the potential severity of the spade bit, chains added to the ends of the reins to balance the bit in the horse’s mouth, and knotted and braided rawhide reins which prevented the reins from swinging unnecessarily, even at a lope, the “made” reining horse seemed to run, stop, spin and handle a cow on its own, with little communication from its rider.
Today, reined cow horse events are “open” to all breeds and held by the National Reined Cow Horse Association (NRCHA). The horse is required to perform two or three different sorts of work in one or two sessions. One session consists of reining work, where a reining pattern is performed. This is often referred to as the “dry work.” The other is the cow work, where a single cow is released into the arena and the horse is asked to first hold the cow at one end of the arena (known as “boxing”) then run the horse along the rail of the arena, turning it back without the aid of the fence (known as “fencing”). Lastly, the horse maneuvers the cow into the center of the arena and cause the cow to circle in a tight circle in each direction (known as “circling”). All this must be accomplished before the cow is exhausted. In three event competition, a “Herd Work” session is also included. The herd work is similar to cutting where a single cow is “cut” from a herd of cattle and prevented from returning to the herd by the intervention of the horse and rider.
Source material: https://en.wikipedia.org/wiki/Working_cow_horse