Simply put, collection is when a horse is encouraged to elevate his back and drive with impulsion from his hind quarters. Normally the only time that a horse does this naturally or on his own accord is when he is gaining speed or launching himself in one direction So how is it that we can ask for our horses to work collected? Well first I want the horse soft on the bit and giving at the pole. More importantly is that they are understanding leg pressure. I want my horses to understand that the leg pressure means something beside go. I will start my horses by teaching them that my legs have a pace or cadence and that they need to match that cadence. This way I can ask for not only a walk but a slow or fast walk with out having to give speed control by pulling on the reins. From there I will teach it at a trot and then a lope so that eventually they are comfortable enough with my legs that when I drive up for speed that they will lower their head and respond to the bit because I am driving them to the bit rather than trying to bump their head back to me. Pulling or bumping their head, without using your legs for impulsion causes the opposite reaction than what I am trying to obtain.
Let me explain that more with an example of teaching a horse to not lean or drop a shoulder. Rather than concern myself with the dropped shoulder I will concentrate more on the collection and impulsion. If I continually work on lifting the shoulder by turning his head in and using my inside leg to lift the shoulder up I am not teaching the horse how to lope properly rather I am just temporarily moving him over. If I drive the horses rear end underneath his body causing him to collect and elevate his back, his shoulders will level out and he will lope without leaning. By doing this you will have taught your horse to lope correct with his body centered.
Collection is needed also when teaching your horse to spin and stop. If my horse does not spin collected he will get sluggish on his front feet and eventually fall out of his spins. The reason why is that a horse carries around 60 percent of his weight on his front end. When you collect your horse and teach him to utilize his rear end more you transfer more of his weight onto his hind quarters which makes his front end lighter and easier to maneuver. That is also the reason that most reiners stop on the rear and stay free on the front end. They are collected at a stop and not pulled to a stop.