What does the trigger do on a double french horn?

The trigger on a double French horn has two functions. First, it can help to improve intonation by altering the length of the outer slide tube, which lowers the pitch. This can be especially useful for playing in tune with other instruments or for playing in different keys. Second, the trigger can be used to create different sound effects by altering the resonance of the horn. This can be used to create a more "open" or "closed" sound, which can be useful for different musical styles or passages.