"He has a lovely bascule"
Outside of the horse world, a bascule is a bridge in which one end is counterbalanced on the other.
In the horse world, it's the shape formed by a horse's back as he goes over a jump. A good bascule means that the horse's back is rounded or arched, not hollowed (if a horse jumps with a hollow back it's uncomfortable for the horse, uncomfortable for the rider, and the jump's likely to fall).
This horse has a reasonably good bascule, but could still use to drop his head a little more. Image source: Ronald C. Yochum Jr. via Wikimedia Commons, taken at the Rolex 3 Day Event.