BILL: This is an example of the classic “Impulsion is Energy, Not Speed” concept. While in a pirouette you will see the horse travel more slowly over the ground, in very collected canter he should maintain his rhythm and energy throughout. The whole idea in Collection is that some of the horse’s forward thrusting power is converted into upward lifting power. If you see a pirouette where the tempo markedly decreases and the strides look labored or dragging (as opposed to crisp and active), you’re watching a bad pirouette.
The same concept applies in all the collected work. For instance, inexperienced dressage riders coming from another discipline may sometimes think of slowing down or shortening their horse for a shoulder-in. On the contrary, they should think power, engagement, lift, and expression when the topic of Collection arises. This is what judges want to see, and this is how each exercise will have its intended gymnastic value for the horse.