Power or accuracy
When King Duarte of Portugal came to write his treatise On
Horsemanship, he dedicated a large portion of it to jousting. Controlling your horse was key to winning a joust. In the lists, horses could reach 40 kilometres per hour and managing that momentum could give you a devastating blow but failing to would make you miss your mark. Go too slowly and you will be an easy target for your adversary. Go too fast and you are less likely to set your lance where you mean it.
A well-bred and trained horse was a valuable part of a knight’s state. Death was a risk to the horse as well as the rider in a joust. Overtaxing your horse by riding it too hard could be disastrous. King Henry VIII “ran so freshly and so many courses that one of his best coursers [jousting horses] was dead that night”.
Horsemanship was one of the major skills of a good knight