I came across the saying with brio in Georg Letham: Physician and Murderer, a novel by Ernst Weiss (a contemporary of Kafka's).
The OED gives the following definition of brio: "Liveliness, vivacity, 'go'." (honestly I'm not quite clear on the "go" part of the definition)
Con brio is an Italian musical term meaning with brilliance, liveliness, or spirit.
Apparently, when referring to a horse brio means controlled spirit.