Saint-Saëns, Morceau de concert
“Morceau de concert” simply denotes a concert piece. The French morceau can refer to portion of something, like a bit of food, or morsel. Camille Saint-Saëns (dates) composed this piece, originally called a Fantasie, in 1891 for the horn player Henri Chaussier, who designed the instrument used in this performance.
When listening to the French horn (preferably, simply “horn”) you may hear that some notes within a melodic line sound muted. Horns were a regular part of the orchestra before valves were added to the instrument in the early 19th century. Valveless trumpets and horns both had limited pitches available to them. Horns could change the key they were pitched in by substituting various “crooks”—lengths of tubing that could inserted to change the length of the horn. You can see the photo here showing a natural horn with a collection of crooks.
But crooks only gave the horn a different, still limited, set of pitches. A player could sound additional pitches by changing the placement of his hand inside the bell. Moving the hand further into the bell shortens the resonating length of the horn, thus changing the pitch, but it also muffles the sound. The introduction of valves gave the horn the ability to play all of the notes in the chromatic scale, but fine tuning adjustments still need to be made in certain instances, and the hand inside the bell provides the obvious solution. You won’t normally hear this on a modern “double horn,” which most horn players use, except as a special effect known as “stopped horn” that composers sometimes call for.
Although the Chaussier horn played by Anneke Scott has piston valves, they have different functions from modern valves. She provides a much more detailed explanation of the instrument in an accompanying video here.