We are not experts in this field, however we do read widely and follow the advice of people who are. We cannot recommend the work of the Australian Equine Behaviour Centre (www.aebc.com.au) highly enough in this regard. The system developed at the AEBC is unique in Australia in relying on scientific research to inform the training of horses and riders and does not rely on anthropomorphic concepts such as "leadership", "respect" or "partnership" to explain interactions between horses and riders, including children.
In a nutshell, horses are motivated to try behaviours that will remove pressures on them that they find aversive or uncomfortable. Its technical term is negative reinforcement, which sounds bad, but in fact means that something is removed to motivate the behaviour. Positive reinforcement (often used in dog and dolphin training) means adding something (usually a treat) to motivate the behaviour. We use a form of positive reinforcement, (clicker training) to manage nervous behaviours and to train horses to step over tarps, have their feet handled etc. However most horse training, no matter what terms are used to describe it relies on the application of a pressure (pressure on mouth via bit and reins, pressure on sides with heels, pressure on back with seat bones) to motivate the horse to respond with a behaviour we want( stopping or slowing, moving forward or faster, turning).
What is happening when we apply the pressure? Lets take a rein aid given to a trained horse. We want the horse to slow, so we put pressure on the reins which applies pressure to the horse's mouth via the bit. The horse slows and we release the pressure. By releasing the pressure, the horse comes to understand that the slowing which preceded the release of the pressure was what caused the pressure to stop. The same is true of a go aid-we apply pressure to the sides, the horse moves forward, we release the pressure by removing our heels or ceasing to push. The horse understands that moving forward was what made the pressure on its sides go away and so next time we apply that pressure it should give the same response because it worked last time. If it always works the "relationship" between the horse and its rider will progress quickly and the end result should be a quiet horse that is responsive to light aids in all situations because the horse is never subjected to uncomfortable pressures that it can't relieve.
This basic relationship between a motivating pressure, a response and the removal of that pressure is the basis of all horse training under saddle. Every time we get on a horse we apply pressure. A well trained, quiet and responsive horse is one in which motivating pressures are always removed immediately the horse gives the desired response so that the horse always knows that if it gives the response we have trained, it will be rewarded with the removal or release of a pressure it finds uncomfortable or unpleasant.
A secondary form of training we use with horses is called "classival conditioning". The most famouns example of classical condition is Pavlov's experiment with dogs. In the test, he rang a bell just before feeding the dogs. He noticed that the dogs started salivating after the bell was rung but before they could see or smell the food. He came to the conclusion that the dogs were associating the bell sound with the arrival of food and were salivating in anticipating of receivig the food. He was able to induce the salivation response even without providing the food, although, after many reptitionsin which the dogs did not receive the food after hearing the bell, they stopped salivating in anticpation because the signal (the bell) was no longer linked in the dogs' minds to recieving food.
Horses are very adept at making similar associations within their abilities and we use this to fine tune our training. Voice comands are a perfect example-we use a negativitely reinforced aid-such as our heels to motivate a go faster response and then add a voice command-"trot", to that aid. With many repetitions and provided our timing is right-using the voice command and then giving the aid, the horse will learn to trot from our voice without needing the leg aid.
What has happened is that the horse has learned that the voice command is quickly followed by the negative reinforcing aid of our legs and that by responding to the voice command and trotting, it can avoid the leg aid all together. Classical conditioning allows us to refine our aids, most commonly be training the horse that a light aid will be quickly reinfoced with a stronger aid. Over many repetitions the horse will usually go off the light aid to avoid the stronger pressure. This approach gives us a very powerful tool box for training our horses, however just as with Pavlov's dogs, if the classical cue (the voice command, the light aid) isn't reinforced the relationship between the two fades and the horse may "forget" or ignore the light aid or classical cue. Also, in situations where the horse feels under immediate threat (as in when it is frightened),it will revert to more basic repsonses (such as flight) which can undermine the effectivness of classically conditioned cues. For kids ponies, especially those ridden by beginners, installing clear classical cues, such as voice aids can make the pony much easier to control and can assist the less coordinated riders to get a feel for the the desired responses from their ponies even when they aren't physically able to deliver the aids.
Can Horses do Two Things at Once?
Not really. Horses can only "listen" to one aid at a time and they will listen to the one that is strongest. If it is being told to go (kicks) and stop (reins) at the same time it will try to respond to one of the aids (it can't physically go and stop at the one time) and if the aids are continously applied together it will soon ignore the weaker one (usually the reins and then become hard or heavy in the mouth -see below).
We hijack this "one track mind" to train a horse to enter a float with whip taps. The float is giving an "aid"-that is a "get out of here that's a dark scary place where a lion could be hiding" aid, but our whip tap is giving a "go foward" aid. Generally the horse will find the whip tap more annoying, or more motivating than the fear of the float and when it learns that it can get rid of the whip tap by moving toward the float it can be quickly trained to lead into the float. Provided there is no lion in the float it quickly learns that the float is not a scary place.
A quick word about hard mouths.
We hear a lot about horses with hard mouths and often its as though the hard mouth is the fault of the horse. Except in rare cases, hard mouths in horses are a psychological state, not a physical one, although it is possible to cause severe physical pain and injury to a horse's mouth by using a bit (ANY bit) harshly.
We often retrain ponies with hard mouths and with a little knowledge and some patience it is one of the easiest issues to fix.
Horses develop hard mouths when their riders pull, hang onto the reins, or keep tension in the reins after the horse has given the desired response (stopped, slowed) or if heavy rein aids are used to force a horse into a frame or "onto the bit". By not releasing the rein pressure after the horse has slowed, the horse is not getting rewarded for the slow and it is the release of the pressure that trains the horse, not the pressure on its own. The slowing of the horse's legs becomes decoupled from the pressure on its mouth through the bit and soon the horse ignores that rein pressure because it comes to mean nothing to it, or it can't remove the rein pressure so it habituates to it. Over time, heavier and sharper rein aids are needed to get the slow or stop response, or the horse learns to lean on the bit and is said to pull. Heavier and sharper bits are then fitted, often with an escalation in other behaviour problems such as rearing, jogging, rushing and shying. Many people simply put up with a heavy mouthed horse, believing that it is normal. We don't because heavy mouthed horses usually have other behavioural issues which indicate that it is confused and often in pain.
Horses which bolt despite often severe bit pressure (which is an oppositional response to the aid-that is the horse is going faster instead of slower despite the bit pressure) are often in a panic and concerned only with putting as much distance between themselves and whatever is frightening them.
A quick word about forward ponies
By in large, forward ponies are created and not born. Horses and ponies will vary enormously in their sensitivity to pressure with some needing very little pressure to motivate them to stop and go (usually go) and in general, many horses labeled forward will be on the more sensitive side of the ledger than horses labelled lazy. However, "forward' behavour is usually learned and it is usually the result of several causes. Forward horses will usually go around with their heads in the the air, open their mouths each time the bit is used, swish their tails and move at faster speeds than required, all signs of tension and in effect are attempting to "run away".
At that point, they are not under the "stimulus control" or the aids of their rider. In effect they are fearful. Generally they will continually speed up within a gait, will leap into the next gait without being asked, will often have "hard mouths", instead of going straight they will often curl their necks and move with their bodies crooked instead of slowing, they will often fidget and swing their hindquarters when asked to halt or slow.
These responses can be caused by riders gripping with their legs so giving continual go signals, riders hanging on to their mouths which the pony causes pain which the pony tries to resolve by running away, by nervous riders who won't actually use their legs to give go aids which means the pony loses its habituation to the passive leg pressure of the rider so gives a very big or fast response to a go aid, or the pony has the kind of temperament which is very sensitive to the environment and is easily distracted by wind, other horses, dogs etc.
Forward ponies can develop into bolters, paricularly when ridden by children who don't have the strength to physically slow them down, so it is imperative that any unwanted go responses are immediately deleted with a downward transition. By not letting the pony practice the tense or forward behaviour you are removing it from his repetoire. Don't move on to a faster speed until the pony is calm, relaxed and responds to the aids without fear or anxiety and never be ashamed to go down a pace if the pony is getting agitated or tense in a faster one.