Stop it!
Back when the French Horn started becoming a popular concert instrument, it did not have any valves. Most music was written in arpeggio form (that is, the notes of a chord played in succession) because certain notes of the chord could not be reached without valves. When a written note fell on a pitch that was not within the arpeggio, a horn player would have to “adjust” the pitch it could normally reach to the pitch that was written.
This adjustment was done with the right hand inside the bell. The pitch can be raised or lowered according to the position of the hand; the further out of the bell, the higher the pitch, and the further into the bell, the lower the pitch. Using this adjustment, a horn player could reach all the notes on the scale.
Something very funny happens when a horn is fully stopped (that is, the players hand is inserted completely into the bell, stopping all air from escaping). When stopped, the horn’s pitch does not drop, it instead raises a half step. Many pieces call for “stopped” notes because it makes an interesting sound and can improve the musical experience. The trick is learning to play them precisely.
The technique of playing a stopped note correctly is summed up in this excerpt from an article at the French Horn Information Centre:
In stopping the horn, try to completely cut off the air with the hand. Because of the shape of the hand, this attempt will not succeed; but the closer one can get to complete stoppage, the more clear the stopped tone will become and the steadier the pitch will remain. When the horn is not stopped sufficiently, the result will be a tone which is nothing more than a muffles hoot, and the pitch will be almost uncontrollable and quite sharp.
Strive to get the perfect intonation by learning which point the hand must stop the bell in order to raise the pitch exactly one half step. The farther into the bell the hand is placed when stopping, the higher the pitch.
NOTE : Stopping can only be done on the F horn.
Another thing that can help is purchasing a stopping mute shown here. It cuts off the air perfectly so you don’t have to with your hand. The only problem is, you have to have time to take it in and out of the bell each time, so it’s only worth it when you have long rests in your music.
And remember, each note is raised a half step, so you have to play a half step lower than written when you stop your horn. Now you know, so stop it!