A brass instrument gives one basic note. Changing the pressure with the lips produces a few other notes. Gradually horn players discovered that if they put their right hand in the bell they could change the basic note. This made it possible to play lots of different notes. When Mozart wrote his horn concertos he was writing for a hand horn. When a note is played using the hand in the bell it makes the note sound different: more muffled (a bit like speaking while covering your mouth with your hand). A Mozart horn concerto sounds very different when played on a modern horn.