The term, "sack out", also refers to going to sleep

Sleep

Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

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Sacking out is a method used by horse trainer

Horse trainer

In horse racing, a trainer prepares a horse for races, with responsibility for exercising it, getting it race-ready and determining which races it should enter...

s to desensitize

Desensitization (psychology)

In psychology, desensitization is a process for mitigating the harmful effects of phobias or other disorders. It also occurs when an emotional response is repeatedly evoked in situations in which the action tendency that is associated with the emotion proves irrelevant or unnecessary...

 a horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...

 to potentially frightening situations or objects. It is a process that, done properly, teaches a horse to not fear certain objects or situations, and, over time can be used to teach a horse to stop and listen to its handler in any potentially frightening situation, thus, in effect, to assess the situation instead of immediately acting upon its fight or flight instinct

Fight-or-flight response

The fight-or-flight response was first described by Walter Bradford Cannon....

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An example of the need for desensitizing training is when an object, such as a plastic bag, suddenly blows across the path of horse and rider, which, if the horse has not been properly desensitized, could result in spooking. Spooking is potentially dangerous, as it can result in fallen riders or horses bolting into danger. Another example would be the need to desensitize a horse to the sound of traffic, music, loudspeakers and other stimuli seen at public events. In some cases, a previous bad experience may have given a horse reason to fear a specific situation, requiring a form of Systematic desensitization

Systematic desensitization

Systematic desensitization is a type of behavioral therapy used in the field of psychology to help effectively overcome phobias and other anxiety disorders. More specifically, it is a type of Pavlovian therapy / classical conditioning therapy developed by a South African psychiatrist, Joseph Wolpe...

 to gradually overcome its fear.

A typical method to desensitize the horse to things like plastic bags is to begin in a step by step process with a mildly worrisome object, such as a towel or blanket. A handler reduces the animal's wariness of a strange object by showing it to the horse in increasingly vigorous ways, first by simply allowing the animal to sniff it, then to touch or rub it gently over the animal until it shows no sign of fear, then, ultimately to flap it and wave it about. Through multiple sessions, the horse will first become accustomed to a specific object, then, as the horse is introduced to additional stimuli, including large blankets, plastic bags and other potentially frightening but harmless objects, the animal learns to not fear items that a human handler presents to it.

Because it is impossible to desensitize a horse to every possible source of fear, the ultimate goal is to teach the horse to trust the judgement of its human handler and not react with fright to an unknown stimuli but rather to wait and listen to its handler. By teaching the horse that certain scary objects will not cause it harm, it also learns that a human handler is a trusted leader, and a horse will learn to look to a human handler for safety and security.

There is controversy over various techniques. Some training methods advocate putting only slight pressure on the horse, allowing it to gradually become accustomed to a frightening object, while other methods sometimes advocate techniques that are based on the operant conditioning

Operant conditioning

Operant conditioning is a form of psychological learning during which an individual modifies the occurrence and form of its own behavior due to the association of the behavior with a stimulus...

 principle of flooding

Flooding (psychology)

Flooding is a form of behavior therapy and based on the principles of respondent conditioning. It is sometimes referred to as exposure therapy or prolonged exposure therapy. As a psychotherapeutic technique, it is used to treat phobia and anxiety disorders including post-traumatic stress disorder...

, for example, waving a large blanket on and over a horse tied to a sturdy post so that it cannot escape -- the latter methods often being quicker at first, but also far more dangerous because rapid exposure to frightening stimuli can cause a horse to panic, and, if tied or confined, to risk injury to the animal or handler in an attempt to free itself.